there's absolutely nothing sexy about armpits. that's like saying a knee or an ankle is sexy.
what attracts you to armpits is the aspect of pheromones and an indication of a male partner. it's also why so many dudes into pits hate shaved pits, as a shaved pit doesn't have as strong of a smell, because sweat clings to the hair and traps the smell, thus no hair to trap the smell, less pheromones to get at you, and then the fact that hairy pits are seen as a clear indication of someone being a man, and if you're attracted to men, that's gonna be something that'll attract you.
b.o isn't just to refer to the bad smell of someone sweaty. not everyone is gonna have an adverse reaction to the odor people release when they sweat, for some, it'll be a pleasant smell.
it's all about your brain telling you that you should be horny because you're seeing a man that you wanna fuck