1. Porphyria is of a higher social status than the lover
2. She has taken a lover in defience of traditional ideas of morality
3. She takes the dominant role, coming to the lovers house, placing his head up on her, etc. hence displaying overt sexuality that was considered inappropriate for a woman.
4. When she arrives at the house she removes her outer clothing, which can be interpreted as removing herself from society's constraints upon the individual to be with her lover.
5. She "makes all the cottage warm", carrying out domestic duties showing the affect of society's continual enforcement of roles.
6. The lover realises that although she is with him now, she will be unable to continue to defy society forever, and will once more conform ("she too weak...give herself to me forever")
7. Only through death can the individual be entirely free from society's constraints (she wished to stay with him forever, but "she knew not how her darling one wish would be hear")
8. The males possession of the female, stuffling her independence ("that moment she was mine, mine", the lover killing Porphyria and preserving her "as if she were alive" in order to possess her "forever")
9. Religion, although only tenuously, has been challenged by the last line ("And yet God has not said a word!") as it is commenting on the fact that God does not care about individuals and that there are apparantly no penalties for attempting to stifle the individual. Alternatively, the last line my infer that Porphyria got what she deserved, as she was an overtly sexual woman who took a lover, and so was rightly condemned in the eyes of Victorian society.
I have not heard about the interpretation of the lover as a lesbian, but even if such an interpretation exists it will not significantly add to your arguement unless it connects with your found texts in that way, and besides, although there is no evidence against it, there is no evidence for it either.
That's all I could think of off the top of my head, but I'm sure there are other interpretations that could be used. Try finding scholarly articles or essays to give you some ideas for interpretation, but make sure you make them your own.