The whole idea of Valentines messages is that the recipient does not know who sent the message. There are sites, such as anonymous.to, that specialise in shielding the sender and allow the inclusion of attachments - these could contain viruses or worms.
Valentinesunited.com offers anonymous e-mailing within the body of an e-mail. There is no registration requirement, the message arrives anonymously and the recipient can opt to reply to the sender and initiate an exchange of e-mails. It is up to the original sender whether or not to reveal their identity and then either love will blossom or the recipient can press the button and disengage.
Sites such as these are perfectly innocent but could be misused by virus writers.