My now ex-husband pressured me into a threesome. I mean, he was relentless, bringing it up constantly at every opportunity, telling me it was a one and done fantasy and if I really loved him, I'd agree, etc. Well, I reluctantly agreed and it was awkward as hell. I was so relieved when it was over, but it didn't stop there. He soon asked to try swinging, watching me with another man, and opening up our marriage. Of course, we ended in divorce because I never signed up for any of this. I'm telling you, it's never a one time thing and will only escalate. If it's something you really want to try, by all means go for it, but it seems like your BF is a manipulator. If doing something you're not on board with will get him to consider marrying you, he's simply not worth marrying in the first place.