the subject shifted into a ~risky zone, it was pornography.

Now, why would this be considered a "risky zone" if not for the implicit uncertainty of how people react to certain topics of discussion? Don't hurt yourself trying to answer. It's risky for a number of reasons.

a lady came up to the group and guy A stopped talking.

This lady was not identified as someone well known to Guy A. Indeed, it is assumed that all he knows about her specifically is her gender.

Guy B tells guy A: Are you that ignorant that you are assuming things about this lady?

Immediately, we see Guy B insulting Guy A and assuming that Guy A is ignorant of something. Actually, why should ignorance be grounds for assumption as Guy B is clearly insisting? Thus, Guy B is also guilty of terrible logic.

Are you assuming that she is not adult enough to be able to participate in an adult conversation about whatever the topic it might be?

So what if he is? It's called "discretion". Fools, like Guy B, rush in where even angels fear to tread. But you must also consider that the given assumption is not the only possible assumption. Someone as arrogantly cocksure as Guy B won't have to consider it, but a rational human would. Also, being an "adult" has nothing to do with willingness to discuss a particular topic. Most of the really mature people I know would be quite disgusted with such a discussion, with the possible exception of condemning it out of hand - and how much of a discussion does that take?

Are you not giving her a chance of a doubt that she might actually find this debate interesting and stimulating?

First assumption is that the conversation actually is interesting and stimulating, much less would be so to any and all adults. Second is the continuing assumption that no mature person would be offended, and that no offended people would register a harrassment suit, and that... You get the picture.

Maybe she'd have some interesting points she'd like to contribute.

And maybe her points are really stupid. Who can say? But Guy B likes to assume that they would be so that he can foolishly try to persecute Guy A. Or maybe she'd be really upset. Or maybe she'd been raped. Or maybe she'd been a porn star. Or maybe she's a devout Muslim. Etcetera.

Why are you denying her this chance?

Do we really have to go over this? Why is Guy B assuming that Guy A feels comfortable discussing such subjects in front of women? Or if he assumes otherwise or makes no assumptions, why doesn't he at least consider that possibility? And if he does consider it, but discounts it, why? That's very disrespectful. Because Guy B is an insensitive pratt, that's why.

The point is there are way too many variables for Guy B to be at all justified in his spontaneous rant. He must die.

And for supporting him, you are a fool. ;-)