Let's see if I can decode the issue at hand here:
Skine is complaining because Tai's "so that's why you're always late for work!" comes directly following Marten's complaint that her farting made him lose his boner--rather than the initial reveal that he and Dora are in the shower together. Skine is concerned that this makes it sound like Tai's farting and the subsequent boner-loss is what's somehow making him late.
Forum-goers think that Skine is trolling because it's obvious that Tai's talking about shower sex, and implying that Marten's late because rather than quickly getting ready, he wastes time/gets carried away boning Dora. However, that IS a strange place for Tai to level her accusation, flow-of-dialogue-wise--it really does sound like it's Marten's boner-loss that makes him late.
However, reading the comic again, I'm gonna say that Jeph's thinking was that Marten mentioning his boner is the first concrete proof Tai has that he and Dora are indeed up to sexytimes in the shower. It is also the first time he makes his presence undeniably known. Now, usually Tai is not that naive, and should have gotten from the "we" that both of them are in there, and that they're sexing. But perhaps she was still too sleep-addled to think straight--certainly her weird "it's ok, I won't look, just gonna go potty" insistence makes more sense if she's operating on the mistaken assumption that Dora's just taking a shower by herself, or she and Marten are taking a shower together, but are simply occupied in getting clean and conserving water or whatever.
When Marten speaks up, she finally gets what's going on, and thus makes her accusation re: Marten's lateness habits at that point.
What she said. Well, minus 'sexytimes.'
I think there is one further caveat. Skine seems to be operating under the idea (most straight guys would have this perception, I think) that if a man and woman are sharing a shower, they
must be screwing, not just showering. Otherwise, why shower in the first place? (kidding, kidding) If you assume that perception to be Tai's, too, the whole strip falls apart, reality unzips, and we all fall into the gaping mouth of meaninglessness. OTOH, if we go with the idea that Tai's thinking the way JackFaerie suggests, the comic makes sense.
I don't know many lesbians (or at least, if I know them, I don't know they're lesbians—'its a deep-South thing, plus being NOMGB*), but I can believe they might not think exactly like straight men. Who do think, btw, common myths notwithstanding.
*None Of My Goddamn Business—also a deep-South thing, but not adhered to as much, sadly.
Edit: that should be "Otherwise, why shower
together in the first place?" but it wasn't as funny. I am not the man to let the opportunity to make a lame joke pass me by.