I don't think that's a taboo subject at all! I've actually seen it expressed around here in other threads. Some people say they have compersion, others talk about a cuckhold fetish, I think it depends on how the situation comes about.
Personally, I find it really hot the idea of either of my guys being with someone else. DH has expressed that he enjoys being able to share comments about more intimate things with another but he and DC fluctuate. Sometimes they are all teasing and kind of turned on about me being with the other, sometimes they feel more like saying "TMI!"
To be honest, and I don't think I've mentioned this before but, I had a visit just before X Mas to see DC, it was our first visit and so we were all nervous and aware of it going to become intimate so we had a code in place for the first time we had sex. That was a bit unnerving, but after that, it was all relaxed. DC and I had four days together and well, the last day we did nothing physically because he was, sore.
When I got home that came up, and now both DH and DC will tease me about how I 'broke his dick'. (I still contend it was not entirely my fault!) So there's times it is a real turn on and the times it's not seem to be times when someone isn't feeling as though they are getting enough private attention. Which I think is natural.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year