definition of "pure"

1. Having a homogeneous or uniform composition; not mixed.

2. Free of dirt, defilement, or pollution.

For love, pure mean you do everything out of love, not for political, social or monetary reason etc. Physical intimacy is the nature result of love, having physical relationship does not make love relationship unpure.

On the other hand, relationships are not pure when polluted with political or social purposes. It does not matter if you go to bed with him/her or not. 

you can have pure relationship with 雞，pure business relationship.