One common theme is adventure. Since Gohan has his Beast form, they might go on adventures together fighting enemies. Another theme could be romance, exploring their relationship deeper.
Gohan is pretty darn strong in the manga. He has moments of extreme power, especially when he unlocks his hidden potential. However, his strength isn't always consistent and depends on various plot developments and training.
The relationship could develop out of a need for companionship. Gohan, having lost many of his friends and family in battles, is in need of someone who can understand his pain. 18, being an android with her own complex past, also longs for someone to be close to. They meet and initially just offer each other a listening ear. As they talk more, they find that they can comfort each other in ways no one else can. This emotional connection then grows into a romantic relationship, and finally, they marry as they realize they don't want to be without each other.