@iamsafu
A straight man gets turned gay from the pleasure of having sex with a gay man.
A heterosexual man, for one reason or another, ends up having sexual acts performed on him by an experienced gay male. His heterosexuality progressively crumbles as he experiences the unique pleasure a same sex partner can give. Include internal conflict and struggle to reconcile what he is experiencing with his believed identity. As things escalate, the man's reluctance and resistance grow weaker and weaker against the mounting pleasure of another man's body. As the post-nut clarity sets in, he recognizes the experience as a sexual awakening.
Trope
138