Just the fact that the speaker in the poem starts off with this line:
How do they do it, the ones who make love
Immediately, the speaker in this poem is conveying that they find it impossible to make love without love; they cannot really fathom making love in any other way. Therefore, they are subtlety disparaging lovemaking that is only physical and not from the heart as well. To the poet, the physical act of sex, is just that, a physical act, but sex with love is lovemaking based on heartfelt care and concern for the other person - and vice-versa.
In her poem, Sharon Olds further indicates that she does not understand how someone could "not love the one who came there with them". In other words, the act of lovemaking is such an intense personal experience to be shared in a loving relationship, that she cannot comprehend how sex without love has any real lasting benefit for either individual involved.
Sharon Olds further indicates that sex without love is in essence a selfish act based on satisfying the desires of the individual. The focus in sex without love is personal gratification, predominantly.
which is the
single body alone in the universe
against its own best time.