I agree with Anon Source. It's not a defect of which people need to be warned beforehand. And gosh, the guys you meet will have plenty of genuine defects of which they will not warn you! You can bet several of them do stinky farts, and they won't be putting that on their profiles - and you may well have reason to wish you'd been warned beforehand of that! Everyone has defects, and generally they don't advertise them on a dating site! But virginity is not a defect. It's simply a piece of personal information - private to you, like your menstrual cycle (and like your menstrual cycle, may become relevant once you are in a close relationship - but is not relevant right away, and not really appropriate to share either).
And I also agree with Pagan - there are weird people out there, and you need to protect yourself. Personal things can come up once you're in a relationship, when you have got to know someone and trust them. It's not a secret like 'Oh, by the way, I've been married before', which understandably might cause a guy to say 'Er... why didn't you tell me??'
Also, you want to get to know someone as a friend first, rather than talking about sex straight away - and talking of virginity is basically talking about sex. I know I've said this before, but I would wait until you know them and trust them enough to want to have a physical relationship with them. And don't feel you have to apologise or give reasons for this being a long time. Lots of women prefer to go slow in relationships, regardless of whether they're a virgin - especially with online dating, as it's common knowledge there are creeps and predators out there and so women should be careful. Set your own boundaries with no apology - make it clear that men have to respect them or they can bugger off!
(Also, like, some men might be rubbish in bed, and so they want a virgin because they can convince a virgin that their performance is normal! You don't want to encourage men like that! ;D)