How we cite our quotes:
"Our maker bids increase, who bids abstain
But our destroyer, foe to God and Man?
Hail wedded love, mysterious law, true source
Of human offspring, sole propriety,
In Paradise of all things common else" (4.748-52)
To justify having sex, Milton cites the "be fruitful and multiply" idea ("bids increase") and suggests that only Satan ("destroyer") or a like-minded person would encourage abstinence. Milton does not just champion sex for its own sake, however; note the reference to "human offspring" and "mysterious law." For Milton, marriage is something incredibly sacred, and it is within its precincts that such pure love can exist.
"Let it suffice thee that thou know'st
Us happy and without love no happiness.
Whatever pure thou in the body enjoy'st
(And pure thou wert created) we enjoy
In eminence and obstacle find none
Of membrane, joint or limb, exclusive bars" (8.620-5)
In response to his question about angelic sexuality, Raphael blushes and says, essentially, that they have some kind of spiritual sex. It's not entirely clear, but the implication is that the physical constraints ("obstacle") of human sex do not apply. It seems like the angels have some kind of physical relationship, but it has a spiritual dimension that is more like a high form of love.
"Carnal desire inflaming. He on Eve
Began to cast lascivious eyes, she him
As wantonly repaid: in lust they burn
Till Adam thus gan Eve to dalliance move" (9.1013-16)
One of the effects of the Forbidden Fruit is that it turns pure love into lustful desire ("in lust they burn"). Milton makes it perfectly clear that is not a good thing, as evident, for example, in the use of "burn," "carnal desire inflaming," "wantonly," and, most importantly, "dalliance." The latter is the same word that Satan uses in 2.819 to refer to his sexual encounter with his daughter, Sin. Yikes!