Another huge problem is unwanted pregnancy. Contraceptives are generally less reliable than most people like to think, so if you're not OK with abortion as the backup option (and certain that you'll actually be able to exercise it), it is a sword of Damocles hanging over any heterosexual relationship between (fertile) people who aren't ready for a baby to pop up.

Admittedly, if you use contraceptives with great care and protective redundancy, they can be extremely reliable. However, I don't know how many people can actually pull this off in a consistently precise and disciplined manner; probably not too many. (Not to mention how badly such unrelenting discipline tends to kill fun.)