The grandmother didn't want the children of opposite sexes in bed together because she was sure they'd "do something dirty".
Didn't the thought of two people of the same gender "doing something dirty" bother her at all?
The movie is set in the 50's which being gay was something one would hide as much as possible and the grandmother was old at this time, so honestly I don't think she worried too much. Cathy and Christopher were the two she seemed worried about that would start up some kind of sexual activity.
"It's better to be hated for who you are than be loved for who you aren't."
You have to keep in mind the times and that the grandmother (Olivia) did not have a mother to give her 'the talk' before her wedding night. Olivia's father loved her very much and she loved him, but he raised her more like a son. Olivia's marriage was consummated by rape and although she learned to be able to tolerate sex with her husband there was never any love or pleasure. Her only sexual experiences were painful, upleasant and made her feel dirty so that is how she came to view all sexual activity.
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