Is sex important in a relationship?

So my fiancé an I have even together 7 years and in the beginning the sex was great. It was often and I never complained. But when he moved in 4 years ago it kind of stopped. Some months we have sex every third day and other months we have sex once a week or so. Is this normal? He tells me sometimes he just isn't in the mood. He gets upset when I try to get him in the mood and says I'm too aggressive and it's a turn off. Sometimes I don't know what to do. Just recently he admitted to me when I asked him to see a dr that he was injured down there when he was youg and it involved 12 stitches so he thinks it's "broken" and that's why and he is afraid the dr will just say the same thing. Is this just another excuse? Should I believe him? I just want to have sex more often so we can also get pregnant but it's hard. Any advice or maybe ideas as to why he may be making excuses?