It doesn't make any difference if your butt is inside the house; if you position it for public viewing then you have exceedded the bounds of good taste and some minor criminal laws.
You should talk to an attorney in PA because you might have a defense if it was not done for lewd reasons. In some states being inside your house might be a defense as well. So you can see the importance of talking to a qualified attorney in your area to see if any of these defenses as well.
Please keep in mind that you will not be forced to reveal why you did what you did. If you were smart and didn't provide a statement to the police, your defense lawyer will have an easier time of it. S/he can advise you if your version helps or not. If it doesn't, then you can choose not to testify.
The prosecution bears the burden of proving each element of the offense including your intent, assuming intent is an element. The complaining witness (alleged victim) can only testify about what was seen. The witness has no way of knowing why you did it. For all he or she knows you could have been trying on new clothes and didn't realize the drapes were open.
People get naked in their houses for all kinds of reasons. I think a good defense attorney can get jurors to focus on the more interesting question: Why the hell was the complaining witness driving by your house so slowly that s/he could see what you were doing? And why did s/he continue to look?