Is viral single-stranded RNA in the absence of reverse transcriptase infectious?
In RNA viruses with a single-stranded genome, this RNA can be positive or negative sense. Positive sense RNA is directly translatable by a ribosome, while a negative strand RNA cannot be directly translated and therefore non-infectious. It either has to be converted to a positive strand RNA with the help of an RdRP (RNA-dependent RNA polymerase, carried in the virion) or reversely transcribed into a DNA which then serves as the template for generation of translatable mRNAs. In either case, this RNA is non-infectious in absence of the necessary proteins.
Note that positive strand RNA has to be propertly capped in order to be translatable. This is also not always the case - some viruses generate or steal the cap from the cellular mRNAs once uncoated in the cell.
Another aspect worth mentioning is that positive-sense retroviruses usually code for their genes in multiple reading frames and make heavy use of splicing to produce their proteins. Thus, directly translating their positive sense RNA is not sufficient for infection.
From Chapter 3 in Viral ecology:
For plus-strand RNA viruses except retroviruses, translation of the viral RNA 
follows immediately after uncoating. The viral RNA extruded from the capsid is then 
used by the host translation machinery for directing protein synthesis (Fig. 9). 
For all other viruses, whether of DNA or RNA genome, the step immediately following 
uncoating is either transcription of the genome yielding functional mRNAs or 
reverse transcription of vRNA yielding proviral DNA (retroviruses).

References:

Viral Ecology
Principles of Virology