It’s probably a good thing that you think about such things - it shows concern for your boyfriend and your future marriage. Not sure if it’s right for your age, but reaching the couple years back with my memory, I think it is.
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According to Canon Law, you’d already be able to marry if your local civil laws allowed.
I can’t give you any official teaching that you wouldn’t already have read on your own, but I can tell you that there’s no command not to have close friends of the opposite sex. People say various things, but they base them on their personal experience, not to mention personal understanding of certain theological or moral intricacies. There are plenty of people who believe one shouldn’t have friends of the opposite gender if married, other than shared friends, or at least never meet up alone. There are people who say it totally doesn’t matter and the whole thing is morally neutral.
Personally, and I insist that this is my personal opinion only, I believe that it depends on the people involved. It depends on the friends and on the spouses (to be). For example, friends who hit on a married person aren’t acting like friends. I think such behaviour waives any “rights” to contact or care they might have. Similarly, if one’s attracted strongly to a person of the opposite sex, while we may sacrifice our lives for them, our spiritual, eternal life must take precedence. Risking a life for friendship is one thing, risking a soul is another.
People experience attraction in different ways. They will have different levels of resistance to this or that type of attraction. Circumstances will matter. Also, the road between feeling it and acting on it is different person from person. It is the individual’s responsibility, but while I wouldn’t like to be paranoid, I wouldn’t take any unjustified non-negligible risks.
Another concern is the non-sexual side of such friendships. Or rather the not-physically-expressedly-sexual one.
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The fact we don’t think we feel anything sexual doesn’t mean we really don’t. It wouldn’t be prudent to ignore empirically verifiable data such as how much time we spent with the friend, how it compares to the time spent with the husband (or boyfriend), how intense it is, what level of “sould kinship” is there.
I can tell you that if I had a girlfriend at the moment (and I don’t because I steel have feelings for my ex), I wouldn’t mind friends, but I think I might wonder about the reasons of her having more male friends than female. I would be understanding of such reasons as getting along better with males than females, or even paying more attention to have male friends out of a past despair or fear of not having enough guys to pick from, although I would be adamant about it being contained within the limits of friendship if she wants a relationship with me. However, I would certainly be worried if they were getting so much time and attention it would make me feel neglected. Certain signs of affection I might be uncomfortable with, depending on the scope of deviation from her normal levels of physical affection, same with flirting or sexuality-related remarks. Again, personalities are different so what’s alarming with one person is pretty much normal with another, but I’d be careful about crossing the lines. I would be worried from seeing my girlfriend have prolonged and continual problems with dealing with attraction to friends or sorting out what’s proper and what’s not, more than from any specific acts or expressions. If she talked about a friend being so nice, caring, standing up for her, nice to talk to, I would ask her if she didn’t feel something more to him and maybe even help her with that (even if it meant losing her). But things like this or that friend looking great, being hot, whatever… Sorry. It may be mental poverty on my part, but I wouldn’t take it. Don’t know about other guys, but I’m sure I’m not isolated on this. I would be similarly hurt and probably end the relationship if I saw friends take up boyfriend roles, even limited. Again, I don’t mean specific behaviours as much as the spirit. I can also tell you that my levels of tolerance would be affected by the levels of love I were feeling from the girl, levels of care, respect, attention. If she were a mature person dedicated to me and respecting me a lot, I would give her the benefit of doubt often without even thinking about things. I would be suspicious about having time for friends but not having any for me. I would be suspicious about favourite friends, and especially about new friends (I’m very tolerant about old friends, but I can be tense about new ones or strangers - e.g. I wouldn’t take well to a girlfriend slow-dancing with a stranger because he asked, while with a good friend she would normally have no second thoughts about throwing herself on the neck of, I doubt I’d care).