When a person is convicted of sexual battery in violation of § 18.2-67.4, attempted sexual battery in violation of subsection C of § 18.2-67.5, a violation of § 18.2-371 involving consensual intercourse with a child, indecent exposure of himself or procuring another to expose himself in violation of § 18.2-387, or a violation of § 18.2-130, and it is alleged in the warrant, information or indictment on which the person is convicted and found by the court or jury trying the case that the person has previously been convicted within the ten-year period immediately preceding the offense charged of two or more of the offenses specified in this section, each such offense occurring on a different date, he shall be guilty of a Class 6 felony.

It includes indecent exposure (§ 18.2-387) but not public masturbation (§ 18.2-387.1).

Of course, public masturbation would fall under indecent exposure as well, so you could just charge him with that and be done with it.