Uh male animals don't go into heat, only females do. WTF is this author thinking haha. Going into heat is basically like getting your period for animals. Male animals respond to the female being in heat to arouse them.
Hmm... Well, that's true. From what I know, only some males go into a heat-like states, like elephants. Bears are not part of the species that go into heat. So, yeah, clear lack of knowledge from the author here.
Whether the author knows that or not, the character just as easily could not know a thing about the mechanics of animals mating. I for one have never particularly cared to find out much about that.
Polar bears typically mate between April and May. I'm guessing that'd be the time for him... except he has no other polar bears to mate with... only those mostly humanoid wolf-eared girls.
This is fantasy, so the author can make up any rules they want especially since we have no evidence yet that the MC is simply a polar bear from Earth or not. But also some animals do go into a rut so maybe they were thinking of that?
My concern is the stupidity of the writer having them calmly ask if they should wash their back, then getting getting embarrassed because of the possible sexual implications after the offer is accepted. Why the hell did you ask in the first place then? And why would accepting the offer be considered sexual harassment? Such awful writing and "comedy" but shouldn't expect more from this kind of comic anyway.