Bhashya
तस्या वेदिरित्यादि सर्वं सामान्यं प्रसिद्धम् ।समिद्धोऽग्निः मध्यतः स्त्रीव्यञ्जनस्य, तौ मुष्कौ अधिशवणफलके इति व्यवहितेन संबध्यते ।वाजपेययाजिनो यावान् लोकः प्रसिद्धः, तावान् विदुषः मैथुनकर्मणो लोकः फलमिति स्तूयते ।तस्मान् वीभत्सा नो कार्येति ।य एवं विद्वानधोपहासं चरति आसां स्त्रीणां सुकृतं वृङ्क्ते आवर्जयति ।अथ पुनः यः वाजपेयसंपत्तिं न जानाति अविद्वान् रेतसो रसतमत्वं च अधोपहासं चरति, आ अस्य स्त्रियः सुकृतम् आवृञ्जते अविदुषः ॥
In plain words
'Her altar,' and so on — this much is already familiar ground. The kindled fire sits at the center of the woman's body; the two testicles correspond, the text tells us, to the two pressing-boards, even though the sentence separates them. Just as vast as the world earned by the Vājapeya sacrifice — that is how vast the world is that this act of union earns for the man who understands it. So there is no room here for squeamishness. The one who knows this and comes to a woman this way draws her merit into himself. But the one who does the same thing in ignorance — not knowing what the seed truly is — loses his own merit to her instead.
Close to the Sanskrit
'Her altar' and so on — all this is generally well known [from elsewhere]. 'The kindled' [is] the fire in the middle of the woman's organ; 'those two testicles' are connected, by a construction placed apart in the sentence, with '[are like] the two boards.' As great, it is well known, as is the world of the performer of the Vājapeya, so great is praised [here] to be the world — the fruit — of the wise man's act of union. Therefore no revulsion should be entertained [toward this teaching]. 'He who, thus knowing, engages in the sport from below, draws to himself' — appropriates — 'the merit of these women.' But again, he who does not understand the attainment of the Vājapeya, [and] is ignorant also of the semen's being the very essence — when he engages in the sport from below, his women draw away his merit — that of the man who does not know.