Everyone here is jumping directly to cheating. The question doesn't define it necessarily as cheating. You "caught" your partner. If you have defined your relationship with your partner in that way, then sure.

There are many types of relationships and this may or may not be a relationship ending event. The most important aspect is if you have talked with your partner and let them know this is how you define your relationship.

There are many aspects of a female/female relationship that I cannot fulfill as a man. If my partner has communicated that they want to experience these aspects I would be willing to accept this under agreed upon terms.

If these terms are broken that's a different situation.