Why Aang and Katara don't use their bending on their enemies' fluids to cast them away?
You mentioned that Aang and Katara are pacifist so they wouldn't use that ability to kill people by bending the water and the air inside them to make them explode or something like that, which is correct. However they could use it to send them away, right?
I believe the reason they don't do so is because it's overly complicated for such a simple task. Why bend people's lung's air if you can simply bend the air around them to hurl them away? It serves the same purpose and it's easier since it's "grasping" and bending the air you can see as opposed to the one within the flesh.
As for Katara, she almost always had water at her disposal and didn't need to use people's fluids.
In season 3:
She once use her own perspiration to escape a prison as she doesn't have water but it's a pain at the moment. It would have been easier if she had waterbended her fluids then, but if she didn't think about it, it's probably because it's not pure enough (bloodbend) so she simply couldn't.
She learns to bloodbend but she only uses it when she has to, because she doesn't have water to bend or to counter her opponent bloodbending.
Then why didn't their opponents use it since they didn't care about killing?
In The Season 3 of Legend of Korra, there's one occurrence of an airbender who airbends Korra's lungs' air to suffocate her. But once again, it's a very powerful bender who does it. So if people don't usually do it it's simply because it's difficult: it demands more power, precision and time. It's easier to simply hurl them away and knock them out against a wall, for example, than to bend the air out of them. Plus you can attack multiple people at once this way.
As for water, once again, since you can't waterbend fluids but only bloodbend (which is thus more difficult) there's only a small number of people who could bloodbend, and for almost all of them it was only during the full moon. So it's also simpler to use the old-style waterbending to fight.
So their opponents simply weren't strong enough to do so and wouldn't have much reason to do it anyway.
TL;DR Why bother to bend enemies' bodies if you can bend the pure water/air (which is simpler) to cast them away or knock them out?