I'm not sure plays - particularly plays as complicated as Shakespeare's - ever have a "main moral". The play is about all sorts of things: dreams, sex, acting, fairies and spirits, magic, love, "true love", pagan rituals (of which "midsummer" was actually one) - and I'm not sure you can argue that it was only trying to say one thing.
But if I had to argue that? I'd argue that it's a play largely about change and transformation. The lovers go into the forest sure of who they love and don't love, and - with a little interference from Puck - everything soon changes. Even at the very end of the play, Demetrius - still under the influence of the flower - marries Helena, whom earlier he hated with such passion that he claimed to be sick when he saw her.
The mechanicals, who are anyway engaged in the business of amateur dramatics (and what could be more transformative than the process of becoming a character?) are broken up by Bottom's transformation into a half-donkey.
And, as many critics argue, Theseus and Hippolyta could be seen in some way to "become" Oberon and Titania in the setting of the magical forest: the parts are often double din production. Not forgetting Puck, who transforms himself (read his first speech!) into stools and cups in order to trick the mortals.
So the main moral of the Dream ... in my reading? Everything, even love, changes. Is that the only interpretation? Definitely not.