If the context is sexual, ie kissing, and a guy gets a hard on us it up to their thinking or just happens?

To explain better, if I’m kissing my boyfriend and he is turned on, is it more likely it’s just because of the kissing or of what it could lead to. And what do guys think about when they’re hard whilst kissing?

If the context is sexual, ie kissing, and a guy gets a hard on us it up to their thinking or just happens?
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