Job interviews need not typically happen in hotel rooms, (which is) a practice that men regard as harmless but which makes some women uncomfortable. 

将a practice这个名词短语改为非限制性定语从句，就可以看出which的先行词是happen in hotel rooms这个不带to的不定式。这种定语从句在夸克《大全》称为句子关系分句 （sentential relative clause), 即which的先行词为主句或主句中的一部分。

去掉which is，则是一个名词词组，作补充性状语，主句中happen in hotel rooms作补充说明。