For monetizing a dating site I have some ideas like I will show some banner ads on that site so I will earn some money from that banners and the other option is that I will keep some subscriptions for the members which they will buy and I will get the profit through that way.
Do you see any successful gambling sites are showing any advertisements ? When you are successful with your concept, you will never need to work on extra income. I'm not against your idea of showing banner advertisements but when you are doing enough promotions, you can get very big earning opportunity from your dating/gambling site itself.
I do see bitcointalk and blockchain.info are showing banners but I believe that is their only way of generating some revenue.
Extra income is very helpful for site owners. For some persons the sites aren't responsible for their full income, they just need to keep it updated and generating income from traffic, while they work on another area.
In case of gambling they don't use banners because it's against the networks politicy of use.