... Many health experts hoped that the medication — Truvada, a combination of two antiviral drugs that has been used to treat H.I.V. since 2004 — would be exuberantly embraced by H.I.V.-negative gay men. Instead, Truvada has been slow to catch on as an H.I.V. preventive in the 18 months since the strategy’s approval by the Food and Drug Administration. In some quarters, the idea that healthy gay men should take a medication to prevent infection — an approach called pre-exposure prophylaxis, or PrEP — has met with hostility or indifference. “It’s gotten tons of attention at H.I.V. meetings as...