Figures from the General Teaching Council for Wales (GTCW) show that female primary school teachers now outnumber their male counterparts by over five to one.
There are concerns that many may be deterred by negative perceptions of men who want to work with children and fears of false allegations.
Iwan Guy, acting director of heads' union NAHT Cymru, said: "Male teachers feel under suspicion more than women, especially at primary level.
"Many of the false allegations made against teachers are against male members of staff and that is definitely off-putting for men who want to teach in primary schools.
"It is a concern because children need male influences and role models. Unfortunately, it's a society thing and there is no quick fix."
Although the Assembly government has developed recruitment events and advertising campaigns to encourage more male applicants, the overall gender gap has grown for the past four years and there are now three times as many women as men registered to teach in Wales.
The trend is reinforced by figures concerning new teachers: just 22 per cent of those who achieved the teaching induction standard last year in primary and secondary education were men, compared to 28 per cent in 2005.
However, Ucas figures suggest that interest in primary teaching among men may be increasing: in the year to February 2010 the number of Welsh male graduates who applied for the PGCE for primary teaching rose from 226 to 360, an increase of 58 per cent.
Gary Brace, chief executive of the GTCW, said: "The widening gap in numbers between male and female teachers is concerning, particularly in the primary sector where the imbalance is greatest.
"It is important that the profession is able to provide good role models for both boys and girls and we hope that efforts to persuade more men to consider teaching as a career are successful. We're encouraged by the recent Ucas figures showing an increase in male applicants for primary teacher training."