I don't think that there's a requirement to have Sex Reassignment Surgery (SRS) to be considered transgender, so in that sense I think sp's point earlier is wrong.
Transphobia is, by definition, an irrational fear of trans people, and so, in that sense, worrying that transgender people are going to render women's sport meaningless by taking over and dominating with minimal effort when there is no evidence that this is happening is pretty transphobic. But that said I see the point in making sure that there is some level of standards and regulation. Andy Murray won't tomorrow change his name to Andrea so that (s)he can play in women's tennis and clean up there, but I can see that he shouldn't necessarily be able to. One suggestion, for example, might be to insist that professional sportspeople cannot compete for some time period if they declare a gender change, and I doubt that you'd need to go that far to kill off any chances of (non-existent, remember!) cynical sportspeople wishing to re-energise their failing careers by pretending to be a woman for a bit, or vice versa.