a. Some people are victims of malware which bring up ads and things like this. Plausible deniability
b. This COULD have been deliberate porn viewing, but there is no way to know WHICH internet connection was used, even if that PC does sometimes use a university connection. Plausible deniability
c. This COULD have been deliberate porn viewing, but by someone else who has used the same PC. Plausible deniability
I think the combination of an understanding supervisor and the difficulty they would have to prove wrong doing means that nothing will happen. As has mentioned above, learn from this experience and take measures to make sure it doesn't happen again. If this happened in a public forum while giving a presentation... AWKWARD !