Lets do a small thought exercise:im a 19m who loves watching people getting fucked by dogs and i would love to see my girlfriend be mounted. how do i introduce her to the idea?
Lets say that your gf was really into watching guys suck old man's dicks (or substitute that with something you are not interested in) and would really like to share this interest with you. Would go to a forum and ask for advice:
"How to get my bf to do this for me, I really want to see it?",
"Who here can guide me in the right direction? To make him interested in the scent of an old penis and drinking the cum of elderly men?"
How would you feel about that?