A virginal bride is desired by many incels due to the desire to avoid baggage such as sexually transmitted diseases, trust issues, and negative comparisons to a girl's previous lovers (who may well have been Chads who refused to commit to her).
This virginal bride is in direct opposition to the mommy gf.
Reasons so many incels want one
A virginal bride is regarded as one who was well-protected by her father and/or not given to impulsive sexual behavior; thus, the prospects of her being a loyal and mentally healthy wife are enhanced. Because Chad has not left his imprint on her, sex with her husband might be viewed as something more unique and intimate by her.
Many incels also argue that they themselves are virgins, so it's only fair that their bride be as well. Feminists will often counter, "There wouldn't be so many non-virginal brides if men weren't sluts too" or "Women don't care if their groom is virginal, so men shouldn't care if their bride is virginal." This ignores that the 80/20 rule works to incels' detriment, not advantage; and that the feminine imperative gives foids a reason to want to seek the kind of man who can have many sexual conquests.