Culture and tourism are BOTH offense and BOTH defense in terms of defending yourself from ideological pressure. The oft-repeated notion that tourism is the offense and culture is the defense applies only to winning a cultural victory, where other people's influence over you doesn't make any difference. It's completely misleading and wrong when applied to ideology pressure, because ideology pressure cares about your influence over them AND their influence over you.
In terms of ideology pressure:
Culture is Offense, because it helps keeps other people's influence over you down, which increases the gap between your influence over them and their influence over you, if yours is higher. If you're Influential over them, but you have so much culture that they're only Exotic over you, they get unhappiness.
Culture is Defense, because it keeps other people's influence over you down, which reduces unhappiness. If somebody is Familiar over you and you're only Exotic over them, you get unhappiness. If you had more culture, so they were only Exotic over you, you wouldn't have that unhappiness.
Tourism is Offense, because it increases your influence over somebody else. If you're both exotic over each other, nobody gets any unhappiness. If you bump up your tourism, and now you're Familiar over them, they get some unhappiness.
Tourism is Defense, because it increases your influence over somebody else, which can close the gap between their influence and yours. If you have no Tourism, you'll be Unknown over everybody else, which means that as soon as somebody with a different ideology becomes exotic over you, you get a bunch of unhappiness. If you had enough tourism to be exotic over them too, you wouldn't get that unhappiness.