Just read this piece by Em & Lo on huffingtonpost.com. They state that there are straight men who feel that it is normal for the passion of sex to cool down in a relationship. There is also the opinion that a man’s libido is not keeping up. Lots of things could lead to less sex, and the best thing to do is ask. Overall, two straight married men give an opinion, and one gay single man gives an opinion. It basically comes down to communication being the best way to address an unsatisfactory sex life.
An interesting read:
Do Men Want Less Sex As a Relationship Progresses?