It depends on the partner.
I've been in relationships where I would not feel comfortable having a MMF or a FFM threesome. I have had both. Sometimes it was really good, other times not. For example with my current wife, I wouldn't have a problem having a threesome with another man. The girl before her we did both MMF and FFM (and even group sex). With her it was perfectly natural to have sex like this. We both had the chemistry to do it. On the other hand, I had a threesome with one girlfriend and another girl and it was a nightmare after. She was really jealous.
Unless you absolutely trust your partner (and they trust you) I recommend having a threesome with a stranger. Having one with a close friend is a recipe for disaster.
The part I enjoy about a threesome is watching, especially if it is a MMF. I really am not into double-penetration. Receiving oral while the other guy is going doggy style is fine. The erotic part is watching your girlfriend have an orgasm with the other man and just being there to suck it all in. It is the same while watching a female perform oral on your girlfriend.
Deep down inside I am a hardcore voyeur. I love watching.