I don't think this is by any means universal and it doesn't need to get hostile.
Proportional systems often force parties to be prepared to cooperate or be left with no influence so even with starkly polarised views on certain issues it doesn't pay to get too hostile about it. To take a concrete example, in Norway the two centre-right centrist parties are classical liberals (Left/Venstre) and Christian democrats (KrF/Christian Peoples Party). The Christian democrats are hardline on abortion by Norwegian standards. It's of course genuinely important to them, but it's also clear that it's gained extra importance to them as one of the few areas where they clearly stand out to their potential voters. Yet they remain on "friendly terms" with the liberals who are one of the most firmly pro choice parties. I'm guessing part of this is that in addition to being aligned on a number of policy areas they're also not competing for the same voter base, so letting the polarisation escalate to hostility serves no purpose for either of them.
But when you have two parties fighting over control in a non-proportional system and you care about power in the short term, it pays for them to both seek towards the other to fight over voters near the centre and to exaggerate the importance of the remaining differences.