Apart from what has already been correctly explained, I'll add the Mexican "label" to this:
In Mexican slang, "andar + -ando/-iendo", depending on the context, may mean "to go around doing something", "to be doing something" and/or "to have the intention of doing something". It seems to be the latter here.
Vulgar slang alert.
These Huevocartoon stories always play with a double meaning of expressions: "Darlas" in Mexico has a meaning by itself; that is why the sentence can be presented in two parts. "Las" there replaces "nalgas", and the expression means to "surrender" oneself for sexual intercourse.
This expression is used for both men an women, but the intended comical effect is stronger when it's applied to a homosexual male, which seems to be the case of this character.
So the first character seems to be making a sexual offer to the second, and when it's not accepted, he changes the meaning of his sentence.
(I think that's the contrary idea Pablo explained, although it's funny how the joke works anyway.)