Well, at least I find it less absurd now, but it may also be due to the fact that there are 1-2 autoimmune diseases in my family, one of which has broken out in me so far.
As for Frank: First of all, the given scenarios are necessary to make the plot possible as it is (Frank's hassitation aside). Personally, I think, and I assume that the idea after which Frank was created is similar: family is more than just the collection of hereditary information that you share with each other. Through acquaintances I have often enough heard of rather bad parents, who take their small children to a pub and go drinking (the parent) as an example. And also here in VN there are some examples of bad parents and families:
Lilith's father
Lilith's and Becca's mother
Abby's biological father
Jessica's family
As for spreading the heredity: it may be that I missed something or didn't understand something correctly, but if I remember it correctly it was that Frank starts to reciprocate Becca and Lilli's feelings (but he doesn't want that to happen super obviously under Abby's nose yet, I'm looking in your direction, Lilli) and at the Becca scene, at least for me, Frank had the option to take the initiative (to the amazement of Becca, who thought she would take the lead). With whom he is still hesitating, but that is not so surprising after all, she has been his little girl all her life:Abby. And at least with me, he was thinking about the quadrangle a few times. Long story short, yes a little hesitant he was but at least with me he wasn't nearly as extreme as in the last update.