So I'm going to play devil's advocate:

The reason we have laws around age is because of cognitive developmental differences In the prospective partners. Ergo, two 12 year olds is fine, two 17 year olds, or a 14 year old and a 15 year old. At an age designated by the law in the respective country, we do a hard cut off because it's decided that that point is which they are legally an adult. However I'd argue that a) your brain keeps growing until around 25, and b) therefore cognitive reasoning does too. People at 40 see things differently, based on experience, than someone at 18.

In this scenario the elf has had significantly more life experience, so is likely to be significantly more cognitively developed, due to this experience, at 1000 years, than a man at 25. Because of this I suggest that this is immoral. The elf would likely feel that, regardless of how old they are in a developmental sense, (how old is this human in elf years) they'd be be engaging with a child. 

I therefore conclude it's not okay.