Is it excusable for a grown up to have a lisp?

People should take responsibility for their own speech. There's nothing wrong with talking with a lisp if you are actually homosexual, but it's homophobic and offensive for a straight person to do it. You should consider the feelings of other people who think you're mocking them before you talk that way. It may seem like a minor thing, but it can be really hurtful to hear people talk that way.