Hey everyone, I am a newbie here so please be easy on me!

So I am not looking for hook-ups or just having sex, I am looking for a long time relationship, which will hopefully turn to marriage in the long run. So I have read some of the theories and if I am not mistaken, on the sexual marketplace women offer sex for men and men give them protection in return. So I have this idea: what if LTRs fail, because men have sex with their girlfriend without marrying her. Because of this, men don't feel obliged to marry her, since all it brings is unnecessary trouble and they get sex anyway. Since we know that women's SMV peak earlier than men's do, what if they are sub-conscientiously feeling unprotected, since the male can leave them anytime he wants. But if they were married, it would bring a lot of trouble to do so. Also if he marries her then he makes a commitment, meaning he will stay with her. We know that during our youth, women are in the peak of their SMV, so Briffault's Law is very relevant in this stage. But as time passes, they loose this power, and men become dominant in this field as well. Women most likely know it, so the best option for them is to get married in their youth. And if they fail to do this with their current boyfriend, they cheat on them.

So this is what I have been thinking about lately. If this is correct, is the best option not to fuck her until you marry her if you want a LTR, else you should be expecting cheating?

Also, if you can, please give me some working advice on LTRs. I would be really glad if you could help me out.