Ask her if she morphs into someone else.

"Um... 'mom'. Could you um... transforms into someone else, like er... anybody?" you asks your shapeshifting girlfriend/mom awkwardly and shyly.

"Of course, sweetie. I could do that." your transformed girlfriend/mother quickly answers. "Who do you want me to be, sweetie? Your teachers, your classmates, your neighbors, your nude playboy models, or your pornstars?"

"I don't know, 'mom'." you reply. "It's up to you, 'mom'. Whoever you are, I would like to have sex with you as always."

"Even if I'm a man, a members of your family, and different age, sweetie?" she asks.

"Yes, 'mom'. Even you're a man, or even someone old enough to be my grandma, I still love you." you reply back to her again as you kiss her once more.

"Teehee, you're always been my weird sweet boy sometimes, sweetie." your girlfriend/mom smiles and kiss you again. "So without further to do, shall we begin?"

With that, your shapeshifting girlfriend/mom gets up and walks back as she's about to morphs herself, you starts to wonder who will she going to be when she morphs into somebody else.