Hi,
Our Lord said that even looking at someone with lust is sinful. So anything done for lust is sinful. The Church defines lust as venereal satisfaction sought for, either outside wedlock or in a manner contrary to laws that govern marital intercourse.
Kissing in itself is not sinful. However, it can be if done for lustful purposes. Only the individual can know what his intention is in such a matter.
Simply stating that one has voluntarily engaged in same-sex genital activity is sufficient in Confession. One is not required to go into detail. If some matter needs clarification, the confessor can ask for it.
Fr. Vincent Serpa, O.P.