Whether or not it's normal is not really what matters. What matters is what the boundaries are in your relationship, and the bigger issues to me would be his lying and hiding the expenses. You need to decide what's acceptable and what the boundaries are for your relationship and deal with that, not what's normal or anything else, because this has to do with what makes you comfortable and safe in the relationship.

It doesn't matter if it's porn or gambling or gaming or something totally benign: He is LYING to you, HIDING things from you, and spending family money on it. IF you want to equate this to a "boys night out", like something he does occasionally, there still wouldn't be lying, cheating, hiding, etc. There would be parameters, right? Something like every Thursday and he spends $100 drinking and eating out with the guys. But see how the boundaries are very clear? You can be okay with him watching porn occasionally, but define what those boundaries are.

It also doesn't sound like the issue to you is the porn either, since you said you're okay with it occasionally. It sounds like you know too the problem is the lying, cheating, and essentially stealing from your family.

You may not be good at confrontation, so maybe you need to write out what you want to discuss in the conversation then try to have a calm, reasonable conversation with him. And remember that a conversation goes two ways. On the one hand, y'all need to compromise and decide boundaries, but he also needs to make amends for his behavior, IMO.