It seems to me that you are quite jealous. Yet your niece is in love with someone. You should be pretty happy for her. You do have other nieces so it's not like you're losing out a ton.
Children should have rights. Human beings all have sexual desires. I've never seen a percentage of children who have sexual desires due to feminist and religious censorship, but I suspect the percentage is quite high.
You should not have reason to have suspicions about this young man unless he gives you reason to. Not all men are pigs (that's feminist dogma), nor should they be treated or suspected as such.