What if the professional dancer is not gay? Should (s)he be forced to dance with a same sex partner?
The professional dancers are, above all, professional. They'd do what was needed for the show.
I think it's appalling that several gay men have been on the show and this has never come up, but then Susan comes on and suddenly it's A Thing. A misogyny thing, perhaps.
It's even more weird because she is infatuated with the chap she's dancing with, he was the one she desperately want to dance with, she's even got a pic of Kevin on her fridge - it's not like there was some undercurrent where she'd asked to dance with a woman and been refused.
That said, I do think in the interests of inclusivity the BBC could have a policy/statement about this, the OP is correct: it would just need to be something like 'We have no plans to introduce same sex couples as this isn't a convention of ballroom dancing. However should a potential contestant feel strongly about this for whatever reason, we would of course give careful consideration to how it would work, and take advice from our experts.'.
It's not even a sexual attraction thing, either - this is a bit of a straw man, but what about a very religious contestant who's religion forbade touching the opposite sex when you're not married? No one in the public eye is springing to mind but still. It's an equalities thing to at least be prepared to think about it.