There's likely a subset of men who wear condoms when sleeping around so they can decrease their risk of babies (a measure so they don't have to trust that the woman they're with is both on the pill and taking it regularly). They might not wear them if this contraceptive decreases pregnancy risk in a similar way. STDs may not seem risky enough to merit condoms for a portion of this subset.

This smaller subset would be at higher risk of STDs if they adopt use of this contraceptive and stop condom use while still sleeping around. If the trend continued, the incidence of STDs would increase, which would change the risk factor of STDs, which would sway a subset back into condom use.

All is balance.