Would you leave your girlfriend/boyfriend if they decided they wanted to wait till engagement/marriage to have sex?
No. Exactly the opposite.
No keep them but lie to them and have sex outside of the relationship.
Realistically - yes. Marriage is not an important goal to me, and I wouldn't risk my sexual or relationship health. I'd likely become resentful. Though, if they are okay with my having other physical relationships, then it might be okay.
Likely. It's important to me to have a physical relationship before making a lifelong commitment