Pandarus says this in Act III Scene 1 of Troilus and Cressida.
Pandarus (who has been acting as the sexual go-between between
Troilus and Cressida and who is basically a pimp) has just finished
singing an incredibly obscene and filthy song to Paris, who is
dallying with Helen of Troy in the Trojan palace while the Trojan
warriors are out on the battlefield fighting the Greeks. Paris says
"He eats nothing but doves, love, and that breeds hot blood, and
hot blood begets hot thoughts, and hot thoughts beget hot deeds,
and hot deeds is love." Doves, it would seem, are an aphrodisiac.
Pandarus replies "Is this the generation of love? hot blood, hot
thoughts, and hot deeds? Why, they are vipers: is love a generation
of vipers?" He means "Is this what generates love?" He then refers
to a phrase used in the King James Bible, (three times in St.
Matthew's Gospel and once in St. Luke's) a "generation of vipers".
This means in the Bible, either a generation as we would understand
it or the offspring of vipers. But here he has just talked about
the process of generation (hot blood, hot thoughts, and hot deeds)
of love and then asks if the next step is from love to vipers.
This doesn't make much sense unless you remember that vipers are
snakes and think about what someone with an incurably dirty mind
thinks of when you start talking about snakes. Hot blood makes hot
thoughts which makes hot deeds which makes love and that makes . .
. you know what.
It's a filthy play. Don't even try to get started on the song he
sung just before.