If you're looking to seduce a married woman, you may need slightly more than a good love poem - although I suppose it's not a bad place to start.
Love poems to send a married woman I'm personally a big fan of the Romantic poets (a group of English poets of the mid/late 18th century) and I'd certainly recommend that you look at the poems written by John Keats as a starting point.
Much of Keats' poetry was dedicated to the love of his life: Fanny Brawne, and I find his words and imagery intense and simply breath-taking. You can see for yourself, as most of his works are available here.
Another suggestion would be to read up on the work of Keat's contemporary, Lord Byron, who was a fascinating character (you can read all about him here).
Byron wooed many a married woman (including Lady Caroline Ponsonby-Lamb) so using his poetry sounds like a good idea to me.