1)It could be part due to "insecurity" coming from low sexual experience, I use this term cause its familiar but its not the best one, I would consider it more "risk management".
A guy with noe experience undoubtly will provide a worse performance than other guys therefore a girl with experience could compare and decide that he is not a better deal then leave him or simply cheat on him if she likes him but not the sex, so its not insecurity but a subconscious alarm of what it may happen.
2)Ego, at an animalistic level we dont want to share our territory, wheter you like it or not mammals males see their femals are their territory...you win the fights with other males you get them, you lose you dont...there is no lion with a partner but no territory, they go togheter...also do you find a used car as valuable as a new one?
If used cars were as good as new ones why those who can afford new ones just dont save money and get used ones?
3)Instinct, all the researches prove what instinctually we all know, the more sexual partner a female has, the less she is able to bond with a partner and we all know what happens when the bond is weak....arguing, nagging, complaining, cheating, divorce and so on.
Now I would have a relationship with a non virgin girl, but for some particular reasons, I taken the virginity of some girls therefore I had the pleasure to be the first one to put my flag on new lands (pvssies and mouths) and because I dont put too much investment on girls so I can afford to be left or cheated with minor conseguences, finally I dont rely on my self confidence and validation on a woman, Im not a cyborg which doesnt suffer but surely I wouldnt become an alchoolic due to a breakup.