If your partner says they’ll marry you IF you do something they want (but you don’t really want) - it’s a sign to RUN AWAY. it’s also a possible sign of controlling and manipulative behaviour.

Not to mention, it’s disgusting how some people manipulate others using serious decisions and commitment such as marriage.

My answer to this would have been: “Excuse me, sweetie, but I don’t want to have a threesome, nor do I want to share you with anyone. It’s not my cup of tea, and I don’t want to be pressured into it using marriage. If we love each other, we will get married. Also, did I forget to tell you I have no problem proposing to you myself? Yep. You may or may expect a proposal some time soon… or maybe some time in the future. You never know which one of us will pop the question. It’s not a race, but… you know. :) No threesomes though.”
- and if he were to run away because of it, I would surely be happy I dodged a bullet.

I hope you’ll find someone nice and decent.