Title text: I'm leaving you for your twin. He's more mature than you by now.
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 him or her either a really bad partner, or a really good one. 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, the former will only experience a few years while the latter will age decades. Megan (or Cueball if he rolls that way) expresses a preference for the older twin, who will be more "mature", meaning both older and less combative about simultaneity.
- [Cueball and Megan in bed.]
- Cueball: Mmm, simultaneous orgasms.
- Megan: That wasn't simultaneous.
- Cueball: Huh? It totally was!
- A common disagreement when one of you is doing all the moving.