Sigh................
If you have any doubts at all you shouldn't be marrying her.
You really don't want my opinion on this one.. I'm a negative prick.
You seem insecure. Maybe you aren't ready for marriage? Just because being alone all the time is unpleasant, doesn't mean you should compromise and marry just someone to avoid being alone.
I'm 25. There is no way in hell I will ever get married until my penis is the size I need it to be. 11 months ago I told myself I would not even have a girlfriend or have sex until I was desired size. But in that 11 months I have been dedicating my life to becoming a "real man" and now I would have a girlfriend or have sex if the proper situation came about. I am unhappy with the size of my penis but I know that I could still fuck the shit out of a woman. Just nothing that a woman would want to be stuck with for any long amount of time.
I guess what I'm saying to you is I have had these doubts that you have. I used to be a pussy. Doubting of myself. But I am slowly becoming an alpha male. It's a long hard road and in reality, I'm pretty sure I won't be ready for a serious relationship for at lest 3 years. Sometimes you need to stop and work on yourself so that in time all things will be the way you know they can be. Never compromise.
If I was in your situation.... well I would not be engaged to a woman first off. I'd grab her by the hair and tell her marry me right now or leave me. Fuck the Western World and their ways. I'll buy one ring. Or not even get married. Just live together and know that you love each other. But that is a far away dream that may never happen.
If I was with a girl and she was going to go on vacation with her girl friends to a place like Miami, I'd tell her "cool, have fun, I'm going to Vegas for a week with my single male friends. I'll see you when you get back." The point is, obviously you aren't positive she loves you. So in that case, you need to make sure she isn't the only one who is capable of getting meaningless sex. You need to make sure that you are a threat too, that you can go to a bar and pull pussy. It may be the last thing you want to do, but it's necessary. Women need a man that other women want to fuck. They have fragile egos and usually look outward to others to please themselves inward.
Take my opinion with a grain of salt. I am a jaded prick that detests this cultural North American wasteland, full of selfish mannequins stepping on others to please themselves for a quarter second.
The point of my rant is that if you're feeling like this before you even marry her, chances are you are an insecure man and you may very well end up a cuckold. I suggest working on your penis and your body and mind, becoming a strong man that shits where he pleases, even if getting there means years of sacrifice, loneliness and discomfort.
Ditch the woman or tell her exactly how you feel. What's the point wanting to spend your life with someone you can't even be honest with. She'll either lie to you to spare your feelings or give you a genuine reaction. Have some balls. Grab her by the arm, look her in the eye and say "I'm afraid you're going to cheat on me. Prove to me that you love me." Maybe she'll respect you. Not to knock you, but right now asking that kind of question, being suBathmateissive, sitting there worrying and having no power is not something that is respectable. Any man can become a real man. I was once a dickless coward. I am now becoming a man and my testicles have been growing. Good luck to you.