You bring up a very important issue -- the fact that our belief systems can vary from
that of our partners when it comes to sex. Your question brings several thoughts to mind.
Threesomes are normal for people who are interested and consenting to participate within
this type of relationship or sexual activity. Some people are comfortable with these experiences
and others are not. You need to do some soul searching on this topic. Think about how you feel
about being sexual with your friend and your partner. Is this consistent with how you feel about
romantic relationships? How might this affect the way that you feel about your partner and your
best friend? Are you willing to take risks with these relationships? Do you think that all three
of you can have sex for fun and that it will have no effect on your relationships? These are all
questions that that may be helpful to think about.
There is no magic way to "make yourself" feel comfortable with this type of situation. Think about
the questions that I proposed and see how you feel about the answers. If you think that you can consider
the threesome as a fun adventure and that it would be OK with you, go for it!
If you force yourself to engage in this type of activity against your feelings, you may end up being
angry with yourself or losing two important relationships. Talk with your partner about your feelings
and let him know of your concerns.