I guess it is a complex slippery slope.
I don't feel watching a Victoria Secret TV Fashion show to be an infidelity or cheating, even though it might get me thinking about things I shouldn't and it could hurt my wife's feelings if she could read my mind.
I don't think that going to a museum and looking at classical statues or paintings of nude women is an infidelity. And unless I constantly did it, most others would not think so as well.
An occasional strip club visit to watch, talk to, or flirt with a dancer, say once a decade or two? Probably wouldn't make my wife happy, but going there with a buddy or someone's bachelor party, I don't consider it an infidelity.
Watching porn as long as it is not addictive or lessens my ability to provide my wife with the love she needs isn't an infidelity in my mind.
Masturbating without my wife present, watching and/or participating isn't an infidelity in my mind. This is especially true as I am the HD one in an HD/LD relationship. If I were the LD partner and it further decreased my desire for sex with my spouse, then yes it might be an infidelity.
Having intercourse with another woman, that is an infidelity in my mind.
Having a female doctor give me a manual prostate exam? An indignity, but not an infidelity. Having a cute young lady medical technician lube me up to take an ultra-sound of my testicles? Nope, not an infidelity, but pretty arousing. (Yes, that has happened and we were both uncomfortable
) Having a colonoscopy from a female technician? Again, an indignity, but not an infidelity and definitely not arousing!
Falling emotionally in love with someone other than my wife? Definitely an infidelity.
There are many things that can be sexually titillating, sexually arousing, or done for sexual satisfaction (masturbation), that unless they are done so frequently that they interfere with providing your spouse the love, sex, time, and financial resource they need to thrive in the marriage, I would not label as infidelities. There can even be the medically justified, touching of genitals that aren't infidelities.
In short, I don't see any real hard lines, but a slippery slope where intent, repetition and impact on your partner are more important.