Okay, here's where I am prepared for some shade to come my way.
Males - not just human ones - are lecherous by nature. The primary drive of males across all species is to have sex and spread their seed. Evolution doesn't care at all about our social conventions. So yes, males are instinctively primed to be sexually aroused and sexually motivated.
I expect that pretty much all heterosexual male humans will look appraisingly at females they find attractive; I expect that homosexual males do the same to other males.
Most males are self-aware enough that they glance surreptitiously and appreciate inside their minds. Some males lack that self awareness, and will lasciviously ogle a 12-year-old's breasts (and anyone else's they see). Some males lack the wherewithal to keep their sex-related thoughts locked up in their brains, and allow those thoughts to spill out into the world for all to hear. Those males make life supremely uncomfortable for pretty much all females once puberty hits - and I very much appreciate that there are good males out there who will help keep them in check.
I am aware that erections almost never occur on demand (caveat for porn stars, perhaps?). I am also aware that erections can happen based on sexual thoughts. But also happen when the breeze strikes from the right angle.
And I'm also aware that the random inappropriate erections tend to calm down by the early 20s for most normally developed males