Sex is 100% safe during your entire pregnancy unless your doctor finds a medical reason and tell you to stop. Like the other poster said just find positions that are comfortable for you and speak up if it gets too rough. You have a lot of extra blood flow down there so sometimes what felt good before pregnancy might not feel as good during so don't be afraid to tell your husband when something doesn't feel good.
Medically speaking, sex during pregnancy is fine. (Unless of course there is a contra-indication like bleeding or cramping.) It's safe, but you say "my husband keeps having sex with me," so my question is do you feel like he is doing it against your will. Have it if you want it, but don't say yes if you don't. That is the road to feeling used.
Talk to your ob, but mine told me it was fine as long as no spotting or cramping. But maybe avoid laying on stomach just to be safe, and positions that keep you comfortable. If you have concerns your husband should understand, talk to him. Good luck