Put it this way.
If we think of a range from 1 to 100, with 1 being random promiscuity, and 100 being perfect monogamy, then man in general is TOWARDS BUT NOT AT the monogamy end.
The commonest pattern of sexual and reproductive behaviour is a *series* of *almost monogamous* relationships, with a bit on the side, as opportunte. The higher in status, wealth and power the males are, the more they can have on the side. The lower in status the females are, the more they have on the side.
There is absolutely no reason for you to marry her. If you’re already getting sex, then all you’re going to add, is her ability to destroy your life and ruin you.
Don’t do it.
Just string her along, tell her you’re thinking about it, and then if she really pushes you, tell her you will agree on condition that she agrees to you having sex with other women when you want.
Otherwise you will NOT get all the pussy that you want in life, and this will bug you more and more as you get older.