Jeff is a human. He has two arms, two legs, a head, a heart, and all the normal stuff of a human. This includes a penis.
Zhsdfgl is not a human. Zhsdfgl is roughly her species' equivalent of female. She has five radial tentacles, a slug-like foot, no head, and no recognizable internal organs. She has an ovipositer and a weird appendage designed to siphon off her species' males' seed.
Jeff is among the first to arrive at Zhsdfgl's home world. By some miracle, our cultures are similar enough for communication to be established quickly. Jeff and Zhsdfgl are both sexual deviants in their culture and do some, ahem, experimenting. They are initially shunned, but then to everyone's surprise Zhsdfgl lays an egg containing Jeff's offspring.
My question is what, likely, biological reasons might their be for Jeff and Zhsdfgl's offspring to be a fully functioning viable individual?