Mujeriego translates to "womanizer" or "ladies' man." It's a vivid and somewhat provocative word that immediately paints a picture of a certain type of person.
In the song, the singer imagines a scenario where he wishes he were a "mujeriego" and a "hombre malo" (bad man) so that his partner would have a reason for her jealousy. This ironic use of the word makes it particularly interesting, as he's denying being one while acknowledging the stereotype.
A man is being accused of cheating, and he is innocent. His partner keeps hunting for proof that isn't there: she reads a betrayal into any message, insists another woman's perfume is on his skin, and builds a story out of nothing. He isn't angry so much as worn down by it, and the line "me estás matando" is the sound of someone being slowly exhausted by suspicion.
Then the chorus turns the accusation over. "Ojalá fuera cierto" means I wish it were true — he wishes he really had become the womanizer she is convinced he is, that he really did stay out every night and wake up next to strangers. If the crime were real, at least the jealousy would be earned and the punishment would make sense. It's a bitter piece of wishful thinking: being guilty would hurt less than being doubted for nothing.
The last line takes it all back. "Pero no, yo solo tengo ojos pa' ti" — but no, I only have eyes for you. That is the whole tragedy of the song: he is faithful, and it has bought him nothing.
Nodal sings it as a mariachi ballad, with the horns and strings pushing the hurt outward rather than swallowing it, which is what keeps the song from sounding merely defensive.