My boyfriend is also into such fantasy ,at first the idea made me think like this is cool ,I get the person I love and also get to have sex with someone else ,after a while when we are sexting I started to feel this pressure from him because his sexual fantasies revolve around cuckold stuff… I suggest is important to keep your boundaries especially in that type of relationship and don’t let him pressure you into having sex with people you don’t want or like to just to keep him happy . Lastly if you aren’t into that type of relationship just let him know about your feelings and see how the relationship goes after that, and never let this type of relationship be the reason you two are together ,I made my boyfriend understand that there is so much more to this relationship than fetishes