PHY Layer FPGA

If you took a detailed look at the ethernet physical layer of the said network types http://standards.ieee.org/getieee802/802.3.html, you'll recognize, that each uses specific features, that are not provided by available fast serial interfaces of FPGA.

P.S.: Newer Altera FPGA with Gigabit interface are providing a GIGE (Gigabit Ethernet) protocol. As far as I understand, it's (almost ?) equivalent to IEEE 1000BASECX (short-haul copper) that uses unidirectional twisted pairs. Standard 1000BASE-T uses 4 twisted pairs at 250 Mbit each, bidirectionally operated through hybrids.