Not providing a SASE won't prevent your work from being purchased. Your check and letter of acceptance will come in the magazine's own envelope. However, if you don't provide the SASE, you will never get a negative response from a publisher. And that means you'll have to wait several months before you can send the poem to another publisher. Do Your Research A list of publication markets, whether those on our web site or any you find elsewhere, is just a starting point.
Most magazines have their own format, their own needs and preferences.
If you send a free-verse love poem to a magazine that has never published any thing but metered verse on science fiction, you're going to do two things. First, you're going to waste your own time and a bit of postage. Second, you're going to waste the editor's time, and that will probably piss them off. At the very minimum, you should send for those guidelines. They'll give you the basic rules of the house, a starting point a bit beyond a simple list of markets.