FICTION : PEEPING THOMAS <i> by Robert Reeves (Crown: $18.95; 244 pp.)</i>

The age of AIDS has bred some strange hybrids, and “Peeping Thomas,” the second Robert Reeves mystery to feature Boston professor-sleuth Thomas Carlyle Theron, is one of them.

In his first appearance, in “Doubting Thomas,” Theron showed a taste for high jinks and low life in the city’s downtown Combat Zone. Now that sex has become a “blood sport” that scared suburban folks prefer to engage in vicariously, via their VCRs, that old hotbed of sin is cooling down.

Nonetheless, a feminist group that includes Theron’s ex-wife is picketing one of the district’s last X-rated theaters, called, ironically, the First Amendment. Invited, because of his raffish reputation, to escort the protesters as they hunt for an expose allegedly hidden in videos made by a former honor student turned porn queen, Theron witnesses a bomb blast that kills the group’s leader.

“Don’t think sex, think money,” a business-school colleague advises him. And, indeed, Theron’s search for the murderer leads him through publishing houses, faculty meetings, street scams, strip joints and drug-fueled film sessions (all wittily rendered by Reeves, who writes good dialogue) into mortal danger at the hands of the profiteers behind the skin trade.

Reeves could have poked fun at the feminists; he might even have been expected to. That he takes them seriously is admirable, but he goes further: He seems to agree that pornography should be banned; that the free-speech issue is a fig leaf for public indifference; even that, for women, “pornography is proof of (men’s) hate. . . . From the nice guys to the bad guys, from the mama’s boys to the lover boys, they hate us.” True or not, this is gloomy stuff; it casts a pall even on Theron’s reconciliation with his ex, and takes much of the fun out of what promised to be a romp through the demimonde.