What about if you only describe presenters of any sort by how they look if there is something distinctive about it?
I've been to a math talk where the speaker was introduced as the "most buff mathematical physicist in the US". It was probably true - he was also an amateur bodybuilder. Most speakers don't get such an introduction, only the ones with a distinctive (and good) appearance do.
(Similarly, in a CS talk, a speaker was introduced as having "the greatest shirts in all of computing." It was probably true - I've never seen a more blinged out shirt outside of a Filipino nightclub.)
Or, even better, if you know the person personally and know that's how they present themselves, or at least you know they're OK with you calling them "sexy" in front of their spouse. Personal relationships trump general rules.
It's still not a great idea in public scenarios like introducing a speaker at a conference. Completely fine for private scenarios like introducing them to a friend.
As long as you describe all your male presenters by how they look -- then it's not sexist at all.
It depends on how you describe them. Straight men have odd ways of thinking they are describing appearances of men in the same way as women, when in fact they are sexualing the women and then complimenting how sharp a dresser the man is. They give straight male compliments to men, rather than sexualising them.
as I said elsewhere, If you want to see how men can be sexually idolised, look at the gay community. That's men looking at men. Look at the people at gay pride parades, soft core porn or adverts aimed at the gay community (e.g. for gay bars). (Or go to amazon, look at the DVD covers for gay films)
Do you see sharply dressed men with a fine hair cut? No, you see abs, you see topless men, you see bulges in tight briefs, you see wet speedos. This is how you could sexually exploit men's appearance.
If you just act like the cast of Mad Men ("Nice tits Peggy, nice shoes pete"), you have double standards.
As long as you describe all your male presenters by how they look -- then it's not sexist at all.