According to Kratos legend, the Goddess of Kratos obtained her power at age 14, and in accordance to that legend, the goddess can take control of ANY woman aged 14 and up. Sofie, a girl that is kinda close to the emperor as a friend was possessed by the goddess and went on a rampage. Now the main plan of the goddess is literally torment and torture him by removing anyone and everyone important to him so that she's the only one left and so he'd accept her love. As seen in the 2 arcs that are out, the goddess's machinations are pretty horrible. Hades has to burn a town, and murder and entire aristocrat family because of the goddess's plans. In the next arc, its heavily implied that he wipes out the rest of his family via execution because 300 years ago, someone that wasn't part of the royal family was made head so most of the main branch of the modern royal family doesn't have royal blood (Hades being the true successor). This is why Hades is a murderous destroyer 6 years from now.
Here comes Jill. Jill being 10 years old at this point means she's currently out of the grasp of the goddess's powers. Jill also surmises that she has 4 years to first protect Hades' happiness, and to shatter the plans of the goddess. Now, I'm going to gush about how badass Jill is. We all know she's going to throw ship, but she later faces off the goddess in her Spear form, as in, being a literal spear. Jill grabs the spear, and snaps it in two, and throws them back to the Kratos kingdom. To the howling laughter of Rave. Now Hades doesn't actually see Jill sexually at all. In fact once the goddess is gotten rid of, he's perfectly content with waiting for her to grow before anything happens. The underage thing is really because he's really traumatized by the goddess. So yeah, the age thing is actually a plot point and nothing cringy happens. In fact everything is nice because Hades is revealed to have the personality of a house husband, and if anything, Jill will be the breadwinner in any family function between the two.