At the risk of putting everyone off their Wheatabixes, studies have found that (1) men are sexually aroused by the sight of breasts and of erect penises, (2) there is, apparently, an evolutionary explanation for the second (something to do with competing for mates and if one guy got in there you need to also get in there immediately so that your sperm has a chance to beat his to the punch, as it were).
And, (possibly as a result of this) the combination of the two in one person is apparently very arousing. “Trxxxy-porn” is one of the highest searched for types of porn.
Make of that what you will - I assume there’s a certain amount of pseudoscience in there, but it makes a certain amount of sense.