Sure. I know H isn't going to leave me, and I feel pretty strongly that sex can just be sex. It's an open offer in our marriage, but H isn't really interested.
It's not really a threesome IMO if only penetration is happening between two people. What's the other girl doing there if that's the case?
I had a whole two paragraphs typed up but decided to not share TMI. I will say though that if you are ever strongly considering one, don't do it with someone you're close to, someone you see often or someone your friends are close to. It just has the potential to make things weird and complicated.
I wouldn't have one in my marriage though but if I was dating someone then it's a different story. I guess it depends where your relationship "is".
ETA: I don't post here but felt I should chime in lol. Sorry for intruding.