The sexual act does seem to have undue weight.
Look at the scene near the end of GS, where Justine figures out how to work around her and Thomas' "problem." I cheered her for finding a solution, but nevertheless... really, JB? Is it that much of a stupid-easy solution?
And if so, why isn't this stupid-easy solution common knowledge within the house filled with incubi & succubi
Because none of them have been in love? Plus, they were raised to think of love as a poison to be avoided at all costs.
The White Court are like a group of families whose individual members are almost all psychopaths. They don't quite fit the definition of being psychopaths, but it's pretty close. It's the combination of the demon inside of each White Court vamp and how they are raised that makes them what they are.
(Just to be clear, a psychopath can be intelligent, charismatic and very successful in the outside world. I read an article that claimed that perhaps 10-15% of all Fortune 500 company CEO's might qualify as psychopaths. They can fake having empathy and other deep emotions, but they lack real empathy for anyone but themselves. Most psychopaths are not murderers, my guess is a huge percentage are con artists. Such a person has no problems stealing the life savings of an 80 year-old retired school teacher, for example. In fact, they would consider it fun. Sociopaths tend to be introverts and loners, they have difficulty communicating with others, but at times can develop some empathy for an individual or a group. The serial killer Ted Bundy was a psychopath; some people who thought they were his friends told Bundy he should go into politics, that he could become the Governor of his state someday. Ted Kaczynski; AKA the Unabomber, is a sociopath; he left a normal college job and lived by himself in a cabin, but had a degree of empathy for his brother, who turned him in.)
Thomas is obviously an exception to rest of his kin, but according to Lara, there have been others like him. Because of the influence of their demon within them, White Court vampires aren't natural psychopaths; if the word "natural" even applies here, but like Lara and Madeline they can come to revel in being monsters.
So most White Court vampires would never think about how to get around "love immunity." They don't share any empathy with the people they feed off of, Lord Raith didn't have any real empathy for his children, and without empathy love can't exist.
(This topic reminds of a WAG I want to post about Thomas. Unfortunately, I've still got a lot to write so it might take another week or two for me to finish, edit and post it. I think it will draw a reaction, and probably a strong one, from many of you. Unfortunately, lunch is over, time to get back to real work. Maybe I can work on it tonight.)