Dear Katie,
We begin with the truth that the marital (sexual) act must be completed in the natural way.
Foreplay or other physical acts of love and affection are fine between consenting spouses, as long as they do not result in climax outside of genital-genital intercourse. Acts that violate this principle of finality violate the natural law, and are objectively sinful.
Regarding your second question, most orthodox theologians hold that helping the wife to finish is seen as an extension of the one marital act, and is morally acceptable.
Peace,
Paul
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