As a general rule, committed couples -- and particularly married couples -- ought not spend unnecessary alone-time with the opposite sex. Nothing wrong with friendship, or group time, or double dates, or parties, or whatever ... but, going one-on-one is just asking for trouble. That goes double for an "ex" -- someone who you were clearly attracted to at some point.
Temptation will rear its head -- even if its not sexual temptation, there's the temptation to discuss things that you ought to be discussing with your spouse instead. Either way, suspicion will follow. Even just the appearance of impropriety can cause problems ... an acquaintance sees you on what looks like a date with another man, and gabs about it, for instance.
As for enforcing boundaries ... say what you mean, mean what you say, and follow-through. If its a deal-breaker, tell her that. Then, if you must, break-the-deal. Better to discuss boundaries like that before you get married, so at least you have a baseline understanding.
Good boundaries are reinforced by a positive self-image and well-defined values. In a relationship...there is a greta deal of wheeling and dealing...and even some compromise-making on some gray issues, but for the areas that you know are flagrant fouls...you should be able to healthily communicate that such a behavior is unacceptable. Some boundaries are crossed by mistake and you have to be cool-headed...not fliiping out, just being firm to say your peace without whining, guilt-tripping, or other passive aggressive means that show that you are scared of confrontation...because nothing will devalue your respect than that sort of behavior. And if the boundary is being truly tested or completely ignored or disregarded after you can fully say that you made your expectations known...then you don't flip out, you just stand your ground...repeating your expectation in firm control...but at the same time giving the other person the freedom to choose what they want...but if they choose to remain in "this relationship" then they must stop the behavior.
JD; there is only one person you can change, and that is you. So when you say "enforce boundaries", all you can really do is determine what YOU are going to do--you can't "make" your girlfriend do anything. She is what she is, faults and all, and so are you. If she's not as punctual as you wish she were, you can let her know how disappointed you are, ask her to be more considerate of you, and express how upset it makes you when you have nice plans and she comes late---but if that doesn't change anything, then it's a matter of deciding what you're going to do about it---can you put up with this for the rest of your life, or not? Because rest assured that after you get married, it's not going to change.
As for your example about her getting home late IN THE MORNING after working with a "suspicious" male colleague "at his place", that raises so many questions that it's hard to give you a straight forward answer. First of all, unless you're sitting outside her apartment/house staking her out all night, then how would you know what time she got home? But more importantly, what kind of a job "requires" her to spend the night ALONE at a MALE colleague's house ALL NIGHT to meet a deadline? Do you have any idea how wild that is? Even if there were a possibility of her having to work all night with this "suspicious" colleague, it would be at the office or in some public place---and then only if they're third shift workers. Your question is so glaringly off the chart that I'm having a hard time responding to it as only a random hypothetical. If there are that many trust issues going on, then the relationship is doomed from the git-go.
The bottom line is, you can say how you feel, ask that your feelings be respected, compromise where appropriate, and be flexible enough to accept a tolerable amount of differences---but the only way you can "enforce boundaries" is to determine what YOU are and are not willing to put up with, and act accordingly. While you're in the dating/courtship phase of your relationship, that's the time to do that--you really want to know if this is someone you're willing to spend the rest of your life with, and now's the time to find out.
Different people have different relationships with and attitudes towards exes. And exes are different. Most of mine are deeply broken people that don't pose a romantic "threat" to my husband. He and I both acknowledge that bigger temptations are the men I never quite dated. But how to handle the situations is something we work out, out loud, together. I would respect him enough to follow his wish that I not go on a pseudo-date with certain men, and he's mature enough to not feel threatened if others want to meet for coffee.
The same actually goes for late or "late." My family likes previews; my husband doesn't. That could easily affect what's considered "late" for a movie, and neither "side" is "right" or "wrong." The theater can be different, but, again, some people want a chance to read the whole program before the curtain goes up; some people hate milling in the halls. From now on, say, "I want you to arrive by X, so that we're sure to see the whole thing and don't feel rushed. If you are not there by Y, I will go in without you and we can meet after the movie [or I'll expect you to be seated at the first intermission, etc.]."
Lawyers and other professionals routinely work through the night on deadline. We just do it at the office. Are there circumstances under which we can and do work from home? Yes. Would I do work at a male colleague's home all night? No. Could I see a young woman of a different temperament agreeing to it? Yes. Your best bet is to put this one generally, not as your own issue, unlike the others. "That's not normal." "Your co-workers will talk." "For your own sake, you should ask that these late nights move to the office." It's a tricky thing to ask a young woman to change her work situation at the explicit suggestion of a romantic interest.
Anyone wanna guess how I am going to answer this one ??
Wait for it...
Three words for ya, big guy.
Over ... the ... knee
You said "enforcing boundaries in a relationship", which led me (and I imagine nearly all your readers) to conclude that you were talking about a boyfriend-girlfriend relationship, hence the inference about marriage. In fact, in the examples you gave, you SPECIFICALLY SAID "girlfriend". I'm a little confused at this point as to what you are even asking the question for if you're going to ask about "enforcing boundaries" (your words) "in a relationship" (your words) and give examples involving your girlfriend (your words), yet now you're telling us you have no intention of getting married and are not even dating, and now we're jumping to neighbors not turning down their music. One normally dates and has girlfriend-boyfriend relationships because he wants to find an exclusive relationship that may lead to marriage---otherwise, why bother? But the bottom line is this (no pun intended, Michael), whether or not we're talking about a boyfriend/girlfriend relationship...or whatever...you can only change yourself and decide what you, personally are going to do. You really can't make someone else do anything. And if you're NOT in an exclusive relationship of a romantic nature, then how late someone stays someplace else to work with a colleague is flatly none of your business.
Rather than ruminate on my own, I suggest an article here and the associated publication it invokes;
JD you are the only one that can truely answer the question of "boundaries" in this relationship. You know what you see as right or wrong better than anyone else!
In my experience it is always best to be honest both with yourself and your partner. You have to do what you feel is right, if it doesn't bother you that she is late, or having dinner with an ex then let it go. If however you aren't happy with these things then you need to say so, and do it with honesty!
Its no good shouting and throughing a tantrum, or being to direct in the way you say something about it.
You need to do what JD would do, nobody else! Once you have made a decision about how you feel, stick with it don't let other people tell you how to feel or that you are wrong. Be sure to listen to what she has to say and work out the differences.
Hope this helps you, all the best.