Ex 'Bachelor' Sean Lowe -- Smiling Like a Virgin ... Who Just Got Laid

It's official ... Mt. Sean Lowe has erupted -- the ex 'Bachelor' is walking around with a grin that can only mean he finally cleared his pipes by losing his alleged virginity a few weeks after getting married.

Lowe couldn't contain his joy at LAX on Thursday ... exactly 18 days after tying the knot with Catherine Giudici.

The 30-year-old insurance salesman and fitness model claimed he was saving himself for marriage.

So ... 18 days later. Still smiling ear-to-ear? Way to go, Catherine.