You know how some parents don't cut their boys' hair and kept them long like a girl's? And then some of those boys are so androgynous that they look like girls? Winona is one of those parents and Jim is one of those boys. She likes to grow young!Jim's hair long and occasionally style them like a girl's for fun.
Living on Vulcan, Spock doesn't have much opportunities to observe other races so when he meets Jim at a party or something he wasn't able to tell whether Jim is a boy or a girl. It's like you can tell whether an adult poultry bird is a male or a female with a look but can't with young chicks.
Young!Spock became enamored with young!Jim. "I have determined that due to our high compatibility rate that it is only logical for the two of us to bond in the near future." Of course, he meant being engaged but Jim is just like "Aww, you're so cute with your big words that I don't understand but okay. Sure, why not." Thus, Spock proceeds to drag Jim to their parents to tell them of his intent.
--Spock follows Jim around like a besotted selhat to protect him from the troubles that Jim often finds himself in, as well as keeping potential suitors away.