If lesbians are required, the first one that springs to mind is the defunct but often funny badlydrawnkitties.com
Hmm. This is interesting. Let's see what we have. We have results all over the spectrum of exploitation of their sexuality from none to tons.
If exactly one
lesbian is sufficient, there's the... also
defunct 1/0 over at oneoverzero.comicgenesis.com. But it's not exactly light-hearted.
Sounds to me like Exiern.com fits your requirements pretty well, if gender-transformed male counts.
Oh, and El Goonish Shive over at egscomics.com has several lesbians, and is usually light-hearted, and almost always funny.
backwaterplanet.com has some, if xenophilia counts, but it takes a while to get there. Only intermittently light-hearted.
And there must be lesbians somewhere in Buck Godot, Zap Gun for Hire, at http://www.airshipentertainment.com/buc ... e=20070111
Arthur, King of Time and Space has several, and it's pretty light. Funnier if you know the source material at all, but you don't need to be a scholar. arthurkingoftimeandspace.com
Hitmen for Destiny! That's REALLY funny, actually. http://www.webcomicsnation.com/thorsby/ ... series.php
Curvy. It has Boringworld, instead of stupidworld. Very lighthearted, but more focused on sexy than funny. c.urvy.org
Unicorn Jelly. Not really oriented towards being funny, and it isn't lighthearted over most of its run, but awesome in its own way. unicornjelly.com
Now, if you needed gay men
, that would be a challenge. EGS would also work, and AKoTaS too, and I think Curvy at least a little. Early Dinosaur comics established that T-rex and Utahraptor were once gay lovers, but they kind of dropped that. Unicorn jelly also supplies.