Why does Michael choose to stay back and retrieve Rachel rather than go with the Sealand woman?

Michael's primary reason for staying back to retrieve
Rachel is that they love each other. While it is a secondary plot line, the romantic
relationships between David/Rosalind and Michael/Rachel have been mentioned throughout
the novel. While David and Rosalind's relationship is more in the forefront of the
novel, there is a lead up to Michael and Rachel's:

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Rachel's afraid. She's crying inside. She wants
Michael.'

' Did she tell you that?' I
asked.

Petra shook her head. 'No. It was a sort of
behind-think, but I saw it.'

' We'd better not say anything
about it,' I decided. ' It's not our business. A person's behind-thinks aren't really
meant for other people, so we must just pretend not to have noticed them. (Chap
16)

It could also be argued
that Michael understand he and Rachel have a special bond because of their telepathic
powers and the experience of the group. As we saw Michael explain in chapter
10:

Michael
told her it'd be like pretending to have only one arm because the person one wants to
marry has only one arm. It wouldn't be any good -- and you couldn't keep it up,
either.'

Michael understand
Rachel, loves her, and doesn't want her to suffer
alone.

She's
quite alone,' said Michael. 'Would you leave David alone there, or
would David leave you?'

There was no answer to that. (Chap
17)