I would know if he watched it. He is very honest and transparent.He is also a very moral man.
If I was married to a man who watched it I would give him a choice, me or the porn. A man who watches porn is not making his wife feel loved or respected, quite the opposite.
You are skirting the essential question. If you were married to a man who, like many men, denies watching porn; think it's not a big deal because he doesn't do it very often, yet made you feel loved and respected, porn would not affect your marriage...until you found out about his rare porn viewing.
You only know what you know because all signs suggest your knowledge is true.
Moral husband, check. Honest husband, check. Transparency, check. Saves his loving for you, check. None of those behaviors, alone or all together, absolutely reject porn from reaching your husband's eyes ever. Therefore, it is possible that he has seen porn. It is also possible he sometimes sees porn, though only a few minutes when he stops by his brother's house to let the dog out maybe once a month and because your husband doesn't really consider those accidental porn views as watching, and because he saves his loving for you, you feel loved and respected.
If your husband watched and you never knew porn would never affect your marriage.
If your husband watched and rarely wanted sex, porn would affect your marriage.
If your husband watched, yet still wanted you, and you found out he watched, porn would affect your marriage.
Porn only affects a marriage when intimacy is denied, denied due to overload masturbation or denied due to spite for feeling threatened. Porn itself doesn't affect a marriage.