Aufidia’s spouse before, you’re now her lover;
your former rival is the one she wed.
Why want her not as your wife, but another’s?
Does it take fear to make you rise in bed?

[Moechus es Aufidiae, qui vir, Scaevine, fuisti;
Rivalis fuerat qui tuus, ille vir est.
Cur aliena placet tibi, quae tua non placet, uxor?
Numquid securus non potes arrigere?]

Martial (AD c.39-c.103) Spanish Roman poet, satirist, epigrammatist [Marcus Valerius Martialis]

Epigrams [Epigrammata], Book 3, epigram 70 (3.70) (AD 87-88) [tr. McLean (2014)]
    (Source)

"To Scaevinus." (Source (Latin)). Alternate translations:

Aufidia's now gallant, who was her lord!
Her lord thy rival, once again abhorr'd!
Why like another's, nor thine own endure?
Canst feel no fervour, where thou are secure?
[tr. Elphinston (1782); Book 6, part 2, ep. 44]

For years neglected and turn'd off at last,
Dodwell's fair wife upon the town was cast. --
Now Dodwell's coach is ever at her door:
He likes the danger of a common whore.
[tr. Halhead (1793)]

You, Scaevinus, who were recently the husband of Aufidia, are now her gallant; while he who was your rival is now her husband. Why should you take pleasure in her, as the wife of your neighbour, who, as your own wife, gave you no pleasure? Is it that obstacles alone inspire you with ardour?
[tr. Bohn's Classical (1859)]

You are the paramour of Aufidia, and you were, Scaevinus, her husband; he who was your rival is her husband. Why does another man’s wife please you when she as your own does not please you? Is it that when secure you can’t get an erection?
[tr. Ker (1919)]

The wife you divorced, who has married her love,
You're trying again on the sly to recover.
From the fact she's another's fresh charm she derives,
And the danger a zest to adultery gives.
[tr. Pott & Wright (1921)]

First you were the husband
then you became the adulterous seducer
of your former wife Aufidia. And the man
who used to be her seducer is now her husband.
If another man's wife
arouses you
though you cannot respond to your own
can it be the security that keeps you down?
[tr. Bovie (1970)]

You're fucking Aufidia, your ex
who's married to the guy who gave you grounds.
Adultery's the one way you get sex.
you only get a hard-on out of bounds.
[tr. Harrison (1981)]

You are Aufidia's lover, Scaevinus, who used to be her husband. Your rival that was, he is her husband now. Why does a woman attract you as somebody else's wife who doesn't attract you as your own? Can't you rise if there's nothing to be afraid of?
[tr. Shackleton Bailey (1993)]

Your present mistress once you held as wife,
With whom you could not live a wedded life.
Unless provoked by sexual transgression,
You obviously can't achieve erection.
[tr. Wills (2007)]