Agreed with
@Doc Holliday it's the experience. I have done this quite a few times. It's about sharing the girl having extra fun, there will be mm contact (accidental most of the times) and you just need to be ok with your sexual identity and don't let that get in the middle of the experience, just have fun and enjoy the threesome ☺️.
The other thing I will say is , you will spend some time with other 2 adults , and just like SP's do, set the boundaries what's ok what is not and if everyone is OK you will have a great time , with no surprises.
Something else to add... Takes time to feel comfortable with a third person, an analogy use is, imagine you always played tennis solo and then you are playing with a partner. It takes time, everyone needs to take it's turn with the common goal of scoring .