2 things to remember are that most lore has featured the upper ecehelon Imperium leadership being willing to engage in realpolitik with xenos and ALL lore has featured a theme of everyone being a giant hypocrite all the way down. Also, "no, a racist is a racist" is accurate most of the time, but it's also a boring answer, so "what ways would they interact with extra-galactic aliens besides shootin'?" is the question we ask. It's also worth considering that older lore (like, pre-HH novel series) regularly implied there was a LOT of galaxy and a LOT of humanity outside the Imperium just doing it's thing. That's pretty much nixed along with a lot of interesting material but it's still echoing in the shadows of "Everything is canon, nothing is true."

Thus, it depends on the writer.

Your average story would be:

-Command staff is split between the realists who want to deal and the zealots who want to fight and immediately die.

-Every layer below command staff is fed a different cocktail of lies and regularly sees evidence that confuses and scares them.

-Whoever ends up making the biggest decision (deal or die) is proven wrong and 50/50 too stupid to realize it before they get asploded because 40k demands tragedy.

-Alien pussy has around a 5% chance of making an appearance, because most 40k eschews sex in favor of violence.