[ Parenting ] Open Question : Am I over reacting to my son having sexual relationship?

I have a 18 yr old son and he had been dating his girlfriend for a year. I work shift work as a nurse, last week when I was working nights I came home to find that his girlfriend had spent the night. The next morning when i went in to talk to him about it I saw an empty condom wrapper on his night stand. I didn't know what to say so I told my husband thinking he could talk to him about it and he said "well at least he is being careful", yes I agree that is a good point but its still not right. To make matters worse, Last night my husband and I went out to a movie but it as sold out so we came home early and found them on the living room Floor half naked and making out, I;m sure if we hadn't walked in when we did it would have went further. We have had safe sex talks with him before, but I still don't think he should be having sex In MY HOME. and my husband is no help he jsut laughs and says I need to realize he is growing up. Am i over reacting. What should I do