In need of advice

I'm in my mid 20s and my boyfriend and I have been together for almost four years. Two and a half of which has been long distance. We are planning on getting married after we both finish graduate school. This distance has been very hard on both of us. It seems to be hard for him sexually though. I really hate sending naked photos of myself and it makes me unbelievably uncomfortable yet he still asks and begs for me to send them. What should I do?