It's not rare for elite sportsmen to marry very young. They don't have the time and social life that permits them having many love affairs. They usually stick to one woman who will center them and provide stability. You can't compare their lives to a regular person life. The schedule they have is so taxing and demanding that it leaves little time for pastimes like chasing girls (which I'm sure Nole has no shortage of
). But I don't believe Jelena is his first girl, surely she's the first 'official girlfriend' but Novak has been linked to many other girls in the past... out of my head I remember that olympic javalin girl from chile (who was astonishingly beautiful), Masha (don't I'm not sure they had anything going on other than friendship and maybe some flirting), various girls he has appeared with in pics over the internet... who knows, we know little about his real private life.
As for the timing, maybe the sensible thing would be to wait until the season is over, but I don't think that marriage will be negative for him. And Jelena is investing a lot of her years in waiting for Novak, so she may deserve to tie the knot with him. She knows she has to put up with many things that other girlfriends wouldn't put up with, like seeing his guy very few times during the year, living separately for large periods of time, etc.