Because Mohammed spoke highly of the pleasure he got after raping non-consentually exploring love-making with them. (One of his wives was 9 when they consummated... they were married when she was 6.) Muslims thus recognize this pleasure as heavenly, and look forward to participating in the fun themselves.
Of course, not all of them can be expected to have the self-restraint and patience to wait, so they're allowed to gang-rape Western girls (non-believing infidels!). France, Australia, and the Netherlands have been having problems with this lately.