Experienced man, to whom young girlfriends asked for sex, rightly distributed their forces. While he fucks one, the other excites her clit with a toy. Everyone is in business and satisfied. If you change partners, then long over her not have to pant - can quickly cum or jump on top herself. And the girls do not get bored, all the time caressing not only themselves, but also their partner.
What can I say - she did a great job! We had a couple of ladies in our group who thought that it was much easier to pay in kind to the professor than to be up all night cramming incomprehensible formulas and dates. But here, as they say, it is a matter of what you learn!