Uchuu Kaizoku Sara

A princess betrayed and raped. There is also monster rape. 

Two girls getting caught by aliens and became pleasure slaves for the rest of their life. 

Both contain the two main women, who are both very busty in both series', being made into slaves. The sexual assaulter in Aku no Onna is a man, while in Kaizoku Sara it's mostly done by two younger girls. Both are produced by Pixy, and both have very similar art styles. 

Both contain the two main women, who are both very busty in both series', being made into slaves by younger girls. Both are produced by Pixy, and both have very similar art styles (their character designer is probably the same).  

Both are hentais containing one or more girls being trained into becoming slaves by a royal family. Both contain monster rape, humiliation, and magic, though Himekishi Lilia revolves around magic more and uses it to a further extent when it comes to the sexual scenes. They both have a similar feel to them although the set ups for either anime are very different; Himekishi Lilia takes place at a castle in a middle-aged styled world while Uchuu Kaizoku Sara mostly takes place in a spaceship and appears to be set in the future. 

To start it off, girls from each series are extremely hot and that is the only reason needed to watch these series to many. Tentacles are some of the bling, another drawing factor is the humiliation of the victims and the satisfaction shown by the offenders which pretty much sums up the series.