Mark but this flea, and mark in this
How little that which thou deniest me is;
It suck'd me first, and now sucks thee
And in this flea our two bloods mingled be.
Thou know'st that this cannot be said
A sin, nor shame, nor loss of maidenhead;
Yet this enjoys before it woo,
And pamper'd swells with one blood made of two;
He first suggests how since the flea has “suck’d” his blood and hers that their blood should “mingle” … have sex. The flea’s mingling of their blood isn’t a shame, or a sin, or “loss of maidenhead”, and yet it’s doing the same thing  that sex would. Therefore they should have sex because the flea has begun the process, as it were. What the flea has done isn’t shameful and neither would sex be.
The Flea