Could try going to a swingers club. Club 103 in Belmont has daytime events most days (some may be male only you'd have to check) and generally there are events at one or other of the three swingers clubs in Perth each night (two in Belmont, one in Myaree).
You can use one of their play rooms after you pay the entry. Lube, towels and condoms usually supplied.
Some play rooms are private, others are open. (so if your a bit of an exhibitionist you can play while others watch - if you have a pornstar fantasy this may be just the go....alternatively if you're up for some spicier options, you can invite others to join you!!!!!)
Generally, beats the hell out of the hassle of a short term hotel booking. May not be as romantic as a hotel, but a very practical and cost effective alternative with no questions asked, because they expect exactly what you plan to do together, unlike the sideways looks from hotel staff...)
If you are after Perth short stay hotels, there are a number of places that have half day options check out