I've always regarded chastity as a likely implication of paladinhood, but one that in play is almost always easy to adhere to: in the typical D&D campaign, how hard is it not to declare as an action that your PC is sleeping with someone?The Arthurian examples are interesting because they run counter to most D&D Paladin codes, which rarely include stuff like chastity (certainly in later editions), are less opposed to covetousness and so on, and tend to rather focus "being LG".
I did once play a paladin who got married, after a very tortuous and emotionally draining (but non-sexual) relationship with a fellow PC. (This episode also had odd repercussions in real life. The boyfriend of the other player accused me - wrongly - of having an affair with her, apparently simply on the basis that in the game our PCs were romantically entangled.)
I think it's significant that in the real world model for (trad) clerics and paladins, the crusading orders were regarded at the actual time as the best exemplars, and chastity was one of their rules.
On covetousness, the paladin in AD&D has a duty of immediate tithe, plus a permission only to retain sufficient wealth to keep him-/herself and his/her followers in modest comfort. There is also a magic item restriction. (Monks had a similar restriction, but even stricter on magic items. Both classes enjoyed a permission to retain more than the normal amount of wealth in order to fund stronghold construction.)
To the extent that 3E relaxes these requirements, it seems to be taking these classes away from their historical/literary roots. (4e is different in this respect - it leaves the manner of the display of devotion as a matter for player and GM to work out, rather than specifying a code in the class text itself.)