Sounds like there are other things going on here. That you even considered cheating or breaking up brings up some red flags. Maybe you are going through a midlife crisis and can't grip the possibility of being with this one man for the rest of your life. Maybe you aren't ready for marriage. If this no OS issue really is a deal breaker for your wedding, I have to qustion the quality of your relationship. Either your relationship is not healthy enough that it can be jeopardized by something as trivial as no OS, or the no OS issue is just an excuse to cancel the wedding. Either way, you might want to talk these issues out with a counselor before the wedding to find out what really is going on. You mentioned lesbians - do you have a hidden desire to be with a woman? Yes I am serious, and there's nothing wrong with that if you find out that's you. Figure this stuff out before marriage.
I can't tell you what is important to you, but personally I wouldn't want a relationship that wasn't strong enough to overcome my partner's aversion to OS.