I did have this kind of relationship once, when I was in my early twenties. All I can say about it is that you must know each other and yourselves very well, and be extremely mature and objective regarding the possible consequences. However, since you seem to be unaffected by it, I wouldn't worry too much, unless this becomes something that he wants to do constantly and you don't feel happy about it.
In my case, even though I didn't particularly enjoy it and only did it to please my then boyfriend, I did engage in sexual activities with another two men at his behest, which were physically stimulating and very pleasurable and didn't have any emotional significance - which is precisely what I didn't like. Eventually, what he called "a chance to enjoy our youth" became increasingly common, much to my dislike and his frustration - in fact, he became so frustrated that I got the impression that he used these
escapades to spite me, because he felt unloved, unsupported and judged by me for not wanting them (apparently, not respecting my feelings and needs was OK).
Things got to the point where I found out that he had been regularly seeing a married couple and and another two people (a boy and a girl, who didn't know about each other and weren't too happy when they found out, either). Needless to say, I emotionally withdrew from the relationship and started looking for someone else, which apparently
did come as a surprise to him. Luckily enough we didn't live together, so I didn't have any problems with needing to move and disrupting my life, but the break up was rather painful - even if I had been preparing for it for quite some time.
Well, I'll stop bothering you with my sob story. Just use it as a cautionary tale and remember that you don't have to do anything that you don't feel comfortable with... Also, there is a chance that this arrangement might change someone's priorities. Good luck anyway!
