Assuming that's after they formed a relationship and he somehow shares some kind of magic with her, or does Timmy accidentaly turns himself into a MILF and then decides to turn Trixie into her husband. I can work with both but the later needs the instant mind change/identity death some anons where complaining above.
For the first, if someone wants a longer story:
Going from The Boy Who Would Be Queen, Timmy goes to the party as Timantha and strikes a friendship with Trixie. He starts switching back and forth to keep his friendship with Trixie and his normal life, to the point of wishing for secret magical gender changing pills so he doesn't have to rely on his fairies all the time (using a wish muffin so they stay even if he loses his god parents). Eventually he starts to become more and more confortable as a girl and Trixie starts to fall in love with her.
Later in High School, they start a lesbian romance, so he decides to come (somewhat) clean with her and tells her about the pills and ow he wishes to stay a girl with her. Trixie after some angst and some problems with the fact that magic exists, comes back and they continue their relationship, Timmy spending all his time as a girl. His family and friends just assume he got hormones and surgery without them knowing, weirder stuff has happened in that town.
Eventually in college Trixie decides to experiment with the (now unused) pills and turns herself into a guy. Timantha gets pregnant, becomes the MILF in OP, and the kid has two moms (except when MILF Trixie gets kinky).