will your companions become corrupted just by fucking the player or could something else that do it as well?
Just having consensual sex with somebody doesn't increase their Corruption. Even if the PC is hella corrupt themselves, your sexual effluvia isn't mutagenic or anything.
Specific sexual acts might increase a companion's Corruption, like turning Cait into a brainless milk cow or siccing a demon on them, but as So said, that's going to be very much on a case by case basis. In other words, the route each companion has towards Corruption is entirely their own (assuming them have one).