I'd like to say that not only women don't reply to e-mails or flowers; men are just as culpable.
No matter who corresponds with me, I always send back a note of thanks for their interest, and if I am not interested, I say so. It's only polite. But I have sent introductory messages inviting men to look at my profile and see what they think and I have received no response even though I know they have read my message.
It's simply a case of bad manners. No matter how full a mailbox is, it won't kill you to compose a simple sentence or two and cut and paste it to your inbox messages. A basic, "Thank you for your interest, but I am currently corresponding with someone special. Good luck in your search." would be nothing more than an act of kindness.
For those of you who think this is a good idea, cut the above sentence in quotation marks, and use it!
We who use these sites to enable us to meet new people should conduct ourselves in a manner that is appropriate to any social medium.