I would like to know why a lot of men like to see a women turn her self on in front of a man?

My fiancée and I have been together for two and a half years and we have a one year old son together. When we first started dating I would have many orgasms while having sex with him but now it is very rare. The sex is very good but my vagina seems to get dry and my fiancée always ask me what is wrong but I cannot seem to give him an answer.

I am willing to have sex with my boyfriend whenever he wants it, but two minutes after I leave for work he masturbates. I do not understand why he masturbates so much when I am willing to have sex whenever he wants it. He says he has no complaints about our sex life. He also cannot have sex more than one time in a 12 hour period, but he can masturbate 5 times a day. He is only 24. Does he have issues or do I. What can I do about this? I am really uncomfortable with it.