why was irina tied up?

Yeah, she was. Wehrmacht had around 500 military brothels ("Militärbordelle") all across German-occupied Europe in which they kept tens of thousands of sex slaves (at least 34,100). It's estimated that one million pregnancies were the direct result of rapes on the Eastern Front and that it's probable that up to 10 million women were raped. Other sources put it at 10,000,000 instances of rape.

Rape was seen as a weapon of terror and it was also seen as a colonizing strategy by the Germans:

A 1942 Wehrmacht document suggested that the Nazi leadership considered implementing a special policy for the eastern front through which the estimated 750,000 babies born through sexual contact between German soldiers and Russian women (an estimate deemed very conservative), could be identified and claimed to be racially German. (It was suggested that the middle names Friedrich or Luise be added to the birth certificates of male and female babies.) Although the plan was not implemented, such documents suggest that the births that resulted from rapes and other forms of sexual contact were deemed beneficial, increasing the "Aryan" race rather than as adding to the inferior Slavic race. The underlying ideology suggests that German rape and other forms of sexual contact may need to be seen as conforming to a larger military strategy of racial and territorial dominance.