Max is not "protecting" Bradley (selflessly sleeping with a beautiful teenage girl, oh the nobility of his sacrifice for his son! What heroism!).
Max is a mysogynist, a reflection of who Jim once was. He is possessive too, if he wants a woman no one else can have her.
What does she see in Max? Father-figure (at a time when Mum and Brother are at odds). Stacey is a dreamer ("you used to have such dreams" - Jean), and its likely that she does not love Max, but believes and feels as if she does, because her mind (subconscious?) has created it. She believes it because she wants to believe it. Bradley dumped Stacey on the anniversary of Brian's death/Sean's dissapearance, the worst possible time.
Sean is supposed to have stepped into his father's place (Stacey all but asked him to) but is unable to due to his problems with emotion and his feelings about his father.
The rational, normal part of Stacey wants to be rid of Max, and feels guilty and uncomfortable about the affair. The mood swings are the biggest problem, whenever Stacey makes a good decision, another mood swing comes along and reverses it. Stacey is mature and very experienced at looking after others, but is immature (partly due to inexperience) in dealing with her own problems, indeed she seems to prefer avoiding them.
The upshot of his manipulation was this:
I can give you a family life, the life you dream of, something which Bradley and your so-called family will never give you.