(As a side note, snail mating is actual fairly cool-looking: http://www.youtube.com/watch?v=KxejE8HB9lY . I actually like this one of slugs a bit more, skip in to about 2:30 for the good stuff: http://www.youtube.com/watch?v=GSW9kWIRCOQ )
Though if you both happen to be members of the same True Way, starting a conversation about religion and politics can be incredibly beneficial. I know seeing a bunch of Ayn Rand books on a friend's shelf boosted our relationship immensely.
For instance, I am a dog person and my girlfriend is a cat person. Occasionally, we start to debate the relative merits of each. And there may in fact be one which is in general "better"; after all, most people would agree that dog ownership is better than man-eating tiger ownership. But since we both recognize that dog and cat ownership is a rather silly thing to get upset about, we don't.
On the other hand, politics (for example) are considered to be important. And while we may realize that it's nearly impossible to find a "right answer", that our discussion will likely lead to nothing, etc., we don't feel silly getting upset about it. That, combined with our identity-based connection with politics, leads to flared tempers and out-of-control discussions.
That said, I think that HN is generally the wrong place to have these debates. These topics, when treated fairly, require a lot of space and time to flesh out fully, and a tech news forum isn't properly equipped to do that. In addition, they distract from the valuable discussion for which this place is good.