Why would polyps on cervix cause spotting during pregnancy if untouched by intercourse or anything else?

Polyps are "friable": Not in a pan! they are very fragile and are able to grow by producing there own blood vessels which lack supporting structure. As you move the polyps move vwhich even in the absence of direct touch leads to bleeding. They are rarely cancerous and are commonly found because of bleeding even in pregnancy. It is rare for them to be dangerous but the bleeding can be irritating, not usually heavy.

Very friable: Emdocervical polyps bleed easily in pregnancy simply from the engorgement (swelling) that occurs with pregnancy. The polyp wall is thin, erodes easily, and can bleed spontaneously especially if extruding from the cervix. Better to deal with it after delivery unless it causes problems.