Is it ok to have sex in the first trimester?

I have heard of some doctors telling some to abstain from sex/orgasm in the first trimester?  I don't know why this was suggested or what for, could anyone who has had this tell me?  I'm still having sex because quite frankly, my poor hubby needs it and I remember in the first trimester of my last oregnancy we went like a month with no sex because of how awful I was feeling.