I'll try to keep this clean. You sound more reserved and he sounds like a guy whose testosterone runs high. Some guys like that just need to rub one out from time to time and the visual stimulation helps. Some guys are cheating hounds. If you trust your to be the former and wish to understand this better then simply join him. Tell him you want to watch and join but he has to keep his hands above the table (getting the idea?) Also it wouldn't hurt to go with him to an adult toy store and get some toys for each other. Explore this with him rather than be on the opposite side of this or your relationship suffer.