In my experience, men that are emotionally and physically involved with a woman tend to have issues in those sorts of scenarios, even MMF threesomes, and I have been the odd male in more than a few of those.

I personally haven't been plagued by that, as I enjoy seeing my partner being pleasured and joining in, but I have female friends that have chosen to experience it that have not included or informed their spouses or S/O's. In most cases, it seems that is the better option for a woman who is interested.

As an afterthought, I am basically straight in a one-on-one situation, but in the threesomes, foursomes, and moresomes, you do whatever is in front of your face.