This disqualification from marrying a Kohen Gadol is only mid'Rabanan. The Rishonim argue concerning the nature of this Pesul.

(a) RASHI explains that "she is not [considered] a complete Besulah," that is, she is a Besulah with regard to a man, but she is not a Besulah with regard to "Ta'avas Tashmish." Since she has already experienced the feeling of "Ta'avas Tashmish" she is disqualified from marrying a Kohen Gadol because she is considered, by the Rabanan, as a Be'ulah (that is, she will not feel as close to her husband as a total Besulah would feel, for this is the underlying principle of the prohibition of a Be'ulah marrying a Kohen Gadol).

(b) TOSFOS, in the name of the Yesh Mefarshim, and Rashi in Yevamos (76a) explains that women who participate in such activity are disqualified from marrying any Kohen because of the Pesul of "Zonah." This opinion understands these women to be lesbians, and that they were not merely warming each other through bodily contact as Rashi here explains (see RIVAN in Tosfos there). It must be that the Rabanan forbid a Kohen from marrying these women because their actions are very close to Z'nus on a d'Oraisa level.

The Halachah is like the Gemara concludes in Yevamos, that she is not considered a Zonah, nor a Be'ulah (SHULCHAN ARUCH EH 20:2). Nevertheless, it is forbidden mid'Oraisa to be involved in such activity, because it falls into the category of "the ways of the land of Egypt," where women use to marry each other (RAMBAM, Hilchos Isurei Bi'ah 21:8). There is no Malkus for such an act because it is a "Lav sh'b'Klalos" (a prohibition that is not specifically stated, but is learned from a general category), but one will receive Makas Mardus (rabbinically instituted lashes).