Unlike John, Sherlock gets jealous.
And I mean jealous, as in, "Touch John, so much as look at that man with a glimmer in your eye and I'll put you through that wall. Yeah, that one there, the brick one. I will do it, Dimmock. I do not recommend you test me."
Because unlike every other man on earth, Sherlock sees. He sees Dimmock and all the others leering at his colleague, his friend, his date, his boyfriend, his, his, his.
And it's not just the men that Sherlock's worried about. It's all of them, all of the human race (that are legal, of course). John's been with both men and women, he can be pleasured in many ways by both, and he gets along well with women. It's not that ridiculous for Sherlock to get jealous, he has reasons, but it's not like John's going to leave him.
Ever.
But it's not like Sherlock knows that.
Sadly.
John doesn't really get jealous, not all that much. Sherlock is desirable, oh hell yes he is. John's catches glimpses of people checking his boyfriend out, but John is positive Sherlock would never leave or cheat on him.
Mostly.
John's secure in his relationship and trusts Sherlock all the time.
Eh, almost.
"Sherlock, 'It's for a case,' will not make me forgive you for sleeping with someone else."
John knows how sexy Sherlock is. He knows how lucky he is that Sherlock's all his, but he also knows of all the onlookers and the young girls that stare at him and damn well drool.
Really, ladies?
So, I guess it isn't a stretch to say John does get jealous. He, as well as his boyfriend, is very well prepared to put anyone through a wall, you just name which wall.