They do it for the same reason, and in the same manner, that women have welcomed invaders and conquerors since time immemorial:
A female asylum centre worker in Denmark is alleged to have engaged sexually with underage male migrants and filmed the acts, and the staff may have known for months. According to authorities, an unnamed asylum centre worker is accused of engaging in sexual relations with several male underage migrants at the Børnecenter Tullebølle asylum home in the Langeland municipality, reports Danish radio station Radio 24syv.

In at least one case the worker was filmed giving oral sex to one of the migrants, and in another video is said to show the worker engaging in full blown intercourse with a different migrant. All residents of the home are males under 18 years old.

The municipality fired Sisi Eibye, head of its asylum programme, on Monday after evidence obtained by Radio 24syv showed that three members of staff at the asylum home had known since June that the boys at the home were being sexually abused by employees.

A police report showed that some municipal officials had suspicions that at least three of the boys were in a sexual relationship with one of the staff at the centre. In addition, it has been reported that another female staff may have also had sexual relationships with the underage migrants. Authorities have shut down the centre while police investigate the matter.
There are few things as hard-wired into the female psyche as the "don't kill me, I'll have sex with you" response to threatening strangers. That may explain a good part of the "rape fantasy" phenomenon.