Almost  by definition this would be wrong--on two counts:First, adults typically do not have guardians unless they are, at least from a legal point of view, incompetent: in that case, there is a question as to whether consent would be legally valid. "Consent" means more than "agreement" or  "willingness"--it means agreement or willingness expressed by someone who can legally give consent.Second, guardians have legally imposed duties to those they are guardians for. Those duties involve or include acting only in the interest of the one they are a guardian for; however, being involved in a sexual relationship has too much potential for a conflict of interest, for putting the guardian's interests ahead of the other person's etc. It is almost impossible to see a situation where this would not be held to be improper.