Simultaneous
Simultaneous
[Cueball and Megan are in bed.] Cueball: Mmm, simultaneous orgasms. Megan: That wasn't simultaneous. Cueball: Huh? It totally was! [Caption below the panel:] A common disagreement when one of you is doing all the moving.

This comic links to en.wikipedia.org/wiki/Relativity_of_simultaneity .
Cueball and Megan are talking about the sex they just had. Cueball is remarking on how they both achieved orgasm simultaneously, but Megan disagrees.
This comic is a play on Einstein's theory of special relativity . One piece of the theory deals with two observers who are moving at close to the speed of light relative to each other. According to Einstein, events that appear simultaneous to one observer will appear to happen at different times to the other.
So when one partner is moving and the other isn't, it's possible that they experience their orgasm at different times relative to each other. That would require one partner to be moving really fast in one direction, which would make them a really bad partner. [ citation needed ] On small speeds, this effect could not be measured.
The other joke is that partners often disagree with each other — even when the difference is minor and not important to the matter.
The title text is reference to the twin paradox, which arises from another piece of special relativity. In theory, if you stick one twin on a spaceship at near light speed and keep the other back on Earth, and assuming neither accelerates, each will perceive himself to age slowly while the other ages quickly, forming an apparent paradox. Megan expresses a preference for Cueball's older twin, who will be more "mature," meaning both older and presumably less combative about simultaneity. Again, the joke here is that at relativistic speeds, there could be disagreement about which twin is truly the "older" one.
[Cueball and Megan are in bed.] Cueball: Mmm, simultaneous orgasms. Megan: That wasn't simultaneous. Cueball: Huh? It totally was! [Caption below the panel:] A common disagreement when one of you is doing all the moving.
