The muscles there will expand to allow a large log of feces out, so is perfectly capable physically of letting something of similar size in. It is just a matter of not panicking and of getting used to it. Go slowly, get some small dildos and remember it is all in the mind.
It will get less painful - using plenty of lubricant and being very relaxed will help. But if it's really painful and doesn't get any better then neither partner should be prepared to accept that for the other.