Well considering that Utena is about to become what looks to be a demi-goddess, if not outright a full goddess, its quite likely that they could eventually have a daughter.

That or the image could be perfect as plot material for Akio - get a bit of DNA of Utena & Corwin, a cloning chamber, and then 'raise' the daughter as his own - a perfectly loyal one to him, and then set her on her true parents. It's an over-the-top "Evil Overlord" type of plan, but then considering what hell it might put Utena, Corwin, and Anthy through, I could see him doing it.

Or (Okay, I've been watching too much anime and ST:VOY...I've got like 8 ideas now for how a 'daughter' could happen), the good old transporter mixup, or whatever.