It has been scientifically demonstrated that the act of sex causes a bonding effect between people. It is partly chemical so it would even take place with strangers. Hence Someone that has had a lot of partners has desensitized themselves to emotional attachment and the act of breaking those bonds. It follows that they would be less able to bond as deeply as someone who has only had on partner. This last part is speculation but it is not a stretch. The first part has been demonstrated with various studies and research on hormone reactions.
Another reason to prefer virgins is that many STDs can be without symptoms for a time or have a dormant period. Then they can rear their ugly head when you least expect it. Seems like a pretty fair reason to want a virgin to me.
Personally, I think you’re putting way too much faith in that scientific demostration. I believe it’s an individual thing. I’ve sen scientific results believed or debunked here on FR, depending on the desired results.
Myself, if it was truly love, it wouldn’t matter if they had a dormant STD or not.