Sorry I just double checked the walkthrough, you're right it shouldn't make a difference to the ending. (In the original script it did but I changed it so hanging up should only unlock extra scenes during the middle of the game.)
The correct order of choices should be:
(Luka tops Xander)
- "Offer to suck his... (18+)"
- "Cut him off."
- "Stay nude"
- "Leave the two of them alone."
When it gets to the third sex scene the second half of dialogue should change so that Xander let's Luka top him.
If you still have trouble please let me know.