A euphemism for a sex toy or any device intended to enhance sexual stimulation.
The USA has in recent years become the world centre for the coinage of euphemisms. Many these derive from double-talk regarding war and death; for example, 'collateral damage', 'regime change', 'extraordinary rendition' etc. Of course, sex is the other major topic that is replete with euphemisms. The USA doesn't disappoint there either, with recent additions 'light in the loafers', 'chubby chaser', 'carpet muncher' etc. The coy term 'marital aid' is from that same tradition.
The first reference I can find to 'marital aids' in print is from the California newspaper The Valley News And Green Sheet, August 1969, in an advertisement for that oasis of adult entertainment, North Hollywood's Erotica Theater and Bookstore.